JPS60263802A - Optical measuring method of displacement - Google Patents

Abstract

PURPOSE:To obtain an optical and an electric system which both have a fast response speed and take a fast measurement of displacement by using an element of electrooptic crystal, etc., to adjusts the optical path length of reflected light. CONSTITUTION:Light from a laser oscillator 1 is reflected partially by the surface of the wavelength plate 4 of a beam splitter cube BSC2, and ther remainder is reflected by a body 13 to be measured. Those reflected light beams rech a BSC26 through the BSC2 and electrooptic crystal (element varying in refractive index on the lateral and longitudinal axes when impressed with a voltage) and is split into two thereat, and they interfere with each other after passing through polarizing plates 27 and 28. Then, interference signals are converted by photodetectors 29 and 30 into electrical signals, whose difference is amplified by a DC differential amplifier 31. The levels of the interference signals relat to the optical length difference between the two interfering light beams B1 and B2 and a negative and a positive voltage are developed accoring to the displacement of the body 13 to be measure, so the voltage is amplified by an amplifier 32 and fed back to the crystal 25. For the purpose, the relation between the impressed voltage to the crystal 25 and the optical path length difference is calibrated previously and then the displacement of the objective body 13 is obtained by measuring the voltage to the crystal 25.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for optically measuring the displacement of an object to be measured, and in particular to devices that require accurate positioning such as semiconductor manufacturing equipment and coordinate measuring instruments. This invention relates to an optical displacement measurement method used for displacement measurement.

[Background of the invention]

Displacement of the object to be measured? As an optical measurement method, the displacement of the object to be measured is interpreted as a change in the optical path difference, the optical path length is fully feedback controlled so that the optical path difference is always constant, and the total displacement is measured from the feedback control amount at this time. It has been proposed that (Unexamined Japanese Patent Publication No. 58-15850
Publication No. 6) The conventional measurement method uses interference light obtained by irradiating a reference surface and a surface to be measured with optical sound and combining the reflected lights, and then using an electric light provided in the middle of the optical path of the reflected light. The electric flexure element is controlled so that the brightness of the interference light is always maximized or minimized, and the displacement of the surface to be measured is calculated from the control amount of the electric flexure element at this time. Regarding this conventional method, the fourth
This will be explained using figures. First, the displacement of the measured object is small (
(within the operating range of the electric flexure element), the laser beam emitted from the laser oscillator 1 passes through the beam splitter cube (B, S).
A part of this laser beam (CB+) reflected by the 1/4 wavelength plate 4 is reflected by the 1/4 wavelength plate 4, and the other laser beam (B2) is reflected by the surface of the object to be measured 13. reflect. B
l+B2 passes through the beam splitter cube 2 and enters the polarizing beam splitter (P, B, S) 3. B
1 is transparent to P, B, S 3 'e, but B2 is 1/
When the four-wave plate 4 moves back and forth once, the υ vibration plane is perpendicular to the drawing, so it is reflected at P, B, and S. B I
When +82 reaches the polarizing plate 11, Bl and B2 do not interfere because their vibration planes are 90 degrees apart from each other, but only the component in the direction of the transmission axis of the polarizing plate is transmitted, so interference occurs after the polarizing plate. . It depends on the difference in the optical path length between the intensity FiBr of this interference light and B2, that is, the displacement of the object to be measured 13. The intensity of the interference light is converted into an electrical signal by a photodetector 12, and the intensity is converted into an electrical signal by an amplifier 1.
After amplification in step 6, the synchronous detection circuit 17 outputs the high frequency oscillator 14.
Synchronous detection is performed as all output reference signals. This detected signal is passed through a total low-pass filter 18, amplified by a DC amplifier 24, and applied to the electric distortion element 7. In addition, the output of the high frequency oscillator 14 is amplified by the amplifier 15 and applied to the electric flexure element 8 to cause it to vibrate at the full high frequency of the specular lO to always generate an optical path length difference of constant amplitude. Even if the object 13 is displaced and the optical path length difference between B1 and B2 changes, the optical system operates so that the optical path length difference as a whole remains constant. To explain this operation based on FIG. 5, the electric flexi element 7.8
The relationship between the optical path length difference of reflected light and the intensity of interference light when no full voltage is applied to is shown in FIG. 5 (1).

When the full high-frequency voltage is applied to the electric flexure element 8 and the optical path length difference is varied with a constant amplitude Δl, the interference light intensity and the output voltage of the synchronous detection circuit 17 will vary depending on where the operating point is located among a, b, and c. The changes shown in FIGS. 5(II), (■), and (IV) are made. That is, the flux components of the output voltage of the synchronous detection circuit are positive, zero, and negative when the operating points are at a-b and c, respectively. Therefore, the direct current component of the output voltage of the synchronous detection circuit is extracted and amplified by the low-pass filter 18, and the voltage skew element 7
When the object to be measured is displaced and the optical path length difference of the reflected light changes, the electric skew element 7 moves in a direction to compensate for it, and the optical path length difference of the optical system as a whole is always kept constant. .

Therefore, if the relationship between the voltage applied to the electric flexure element 7 and the displacement is calibrated in advance, the displacement of the object to be measured can be determined by measuring the voltage applied to the electric flexure element 7.

Next, a case will be described in which the object to be measured is significantly displaced. Figure 6 (a) shows the relationship between the optical path length difference and the intensity of interference light.
Figure (1) shows the relationship between the optical path length difference and the voltage applied to the electrolytic element 7.The optical path length difference when the object to be measured 13 is at the origin is kXo, and before the device operates. Now, let us assume that the operating point of the device is at point a. When the device operates in this state, the electric flexure element 7 has E as shown in Fig. 6 (b).
A voltage of o is applied, the optical path length difference of Xl-Xo is compensated here, and the operating point moves to point b in FIG. 6(a). From this state, the displacement of the object to be measured increases and the optical path length difference becomes
2, the voltage applied to the electric flexure element 7 changes depending on the size of the displacement meter, as shown in FIG. 6(b). At this time, if the reference voltage kEm of the voltage comparison circuit 190 is set, the output voltage of the voltage comparison circuit 19 is always O, but when the output voltage of the low-pass filter 18 reaches Em, it becomes 1, and the OR circuit 22'(ll-, the wire switch 23 operates. As a result, the DC amplifier 24
The output voltage of becomes 0, and the device is balanced at the 0 point in FIG. 6(a). When the object to be measured 13 is further displaced and the optical path length difference increases, it moves to the balance point na in the same way, and when the optical path length difference becomes X4, a voltage of B4 is applied to the electric flexure element 7 and d
balance on points.

Therefore, the amount of displacement of the object to be measured from the origin position can be determined using the cutting tool 9. The output voltage of the voltage comparison circuit 19 changes every time the object to be measured 13 is displaced by λ/2 (when the optical path length difference is λ)
Each time it increases, it changes from 0 to 1, so the up-down counter 21 counts the positive numbers, and the result is expressed as λ/
2, and then subtract the voltage Fo applied when the object to be measured is at the origin from the voltage E4 applied to the electric force element at that time, and divide this by Em, multiplied by 272, that is, B4-
Just add E0×↓. When the object to be measured is displaced in the opposite direction Em 2 from the origin, the reference voltage k Em of the voltage comparison circuit 20 is set, and the output of the voltage comparison circuit 20 is set to O.
When the value changes from 1 to 1, the up/down counter 21
Count the negative number with . (However, E'4<O) However, the above conventional method has the following drawbacks.

1. The optical system is complicated.

2. Since the optical path length is controlled by mechanically displacing the mirror using an electric flexure element, it responds only at a speed lower than the mechanical resonance frequency of the electric flexure element.

3. As the frequency increases, it becomes difficult to construct a periodic detection circuit, and the maximum speed of displacement that can be measured is limited.

[Purpose of the invention]

An object of the present invention is to provide an optical measurement method in which the optical path length of reflected light from a reference surface and a surface to be measured is completely directly electrically controlled, and the displacement of the surface to be measured is determined from an interference signal generated by the difference in optical path length between the two. It's about doing.

[Summary of the invention]

The optical displacement measurement method according to the present invention includes an optical path length control means that makes full use of elements that irradiate light onto a reference surface and a surface to be measured, and apply a voltage to the optical path of each reflected light to change its refractive index. is provided, an electrical interference signal of opposite phase is generated from the reflected light that has passed through the means, and this electrical interference signal ? Feedback is sent to the optical path length control means to control the optical path length difference of the reflected light so that it is always constant, and the displacement of the surface to be measured is calculated from the control amount at this time. Instead of a b-element, an element such as an electro-optic crystal whose refractive index changes when a voltage is applied is used, and instead of mechanically displacing a mirror to control the optical path length, the refractive index can be changed purely electrically. Since the optical path length is fully controlled by adjusting the optical path length, the optical system is simple and the response speed is fast. Also,
By generating interference signals with opposite phases on two photodetectors and taking the difference, the direction of displacement can be determined without changing the optical path length difference at high frequency with a constant amplitude.
It eliminates the need to handle high frequencies, simplifies the electrical circuit, and improves response speed.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to FIGS. 1 to 3.

In FIG. 1, the same reference numerals as in FIG. 4 represent the same components. 25 is an element whose refractive index changes when a full voltage is applied, such as an electro-optic crystal such as LF' or NbO3. When a full voltage is applied to the electro-optic crystal, the refractive index nr for light having a vibration plane perpendicular to the plane of the paper and the refractive index nu for light having a vibration plane parallel to the plane of the paper change. What is the geometric length of the electro-optic crystal in the optical axis direction? Assuming rJ, the optical path length is generally given by (geometric distance) x (refractive index), so the difference Δ in the optical path length that occurs while the two lights travel through the electro-optic crystal is given by the following equation. .

Δ=Cn, -y'llF>1 Therefore, depending on the applied voltage, P/-G1. By changing the value of , the optical path length difference Δ can be adjusted.

26 is a beam splitter cube (B,
S, C,), 27.28 polarizing plate, 29.30 photodetector, 31 DC differential amplifier, 32 differential amplifier 31
This is a DC amplifier for amplifying the entire output voltage of and applying it to the electro-optic crystal 25.

First, the entire operation when the displacement is small will be explained.

In Figure 3, the light emitted by laser oscillator 1 is reflected by beam splitter cube 2 (B, S, C, 2)! Ru. A part of this light is reflected on the surface of the single-wavelength plate 4 (this light is called 81), and the remaining light reaches the object to be measured 13 and is reflected there (this light is called iB2).

Bl has a vibration plane parallel to the plane of the paper, but B2 has a single-wavelength plate 4.
Is the vibration surface perpendicular to the paper surface because it goes back and forth once? have Both of these lights pass through an electro-optic crystal 25, B, S, and C, 26, where they are split into two, and after passing through polarizing plates 27 and 28, interference occurs. The interference signal is converted into an electrical signal by a photodetector 29.30,
The difference is amplified by the M-flow differential amplifier 31. Here, the intensity of the interference signal, that is, the output voltage of the two photodetectors 29.30, is related to the optical path length difference Δ between the two interfering lights B1 and B2, and this optical path length difference Δ is 4 surface and object to be measured 1
3 optical distance (=optical path length) 11 rA, electro-optic crystal 2
If the optical path length difference that occurs in the crystal between B1 and B2 when the applied m voltage of 5 is zero is Δ0, it is expressed by the following equation.

Δ=21+Δ0 Here, when the object to be measured 13 is displaced by Δl, the optical path length difference expressed by the above equation changes by 2Δl, and the interference light intensity changes accordingly.

Here, if the transmission axes of the polarizing plates 27 and 28 are set at 45° and -45° with respect to the plane of the paper, respectively, the change in the intensity of the interference light after the polarizing plate 27 and the interference light after the polarizing plate 28 will be explained. The intensity changes occur in opposite phases. Therefore, the horizontal axis represents the reflected light Bl
If the optical path length difference Δt between and B2 and the output voltage of the photodetector 29.30 are plotted on the vertical axis, the result will be as shown in FIGS. 2(a) and 2(b). Therefore, when the difference between the output voltages of the photodetectors 29 and 30 is calculated by the DC differential amplifier 31, it becomes as shown in FIG. 2 (c).

Therefore, when no % pressure is applied to the electro-optic crystal 25,
For example, when the operating point is at point a and the optical path length difference between reflected light Bl (!: B, 2 is Xa), the output voltage of the DC differential amplifier 31 is zero, but the operating point is at point b or flc, and the optical path When the length difference is Xb or XC, a positive near voltage and a negative voltage are generated, respectively. When this voltage is amplified by the all-DC amplifier 32 and fed back to the electro-optic crystal 25, the object to be measured 13 is displaced and the optical path length difference is increased. Even if the electro-optic crystal 25 changes, the refractive index of the electro-optic crystal n25 changes, and the optical path length difference becomes Xa, which is balanced.The relationship between the voltage applied to the electro-optic crystal 25 and the optical path length difference that occurs in the crystal is calibrated in advance. The displacement of the object to be measured 13 can be determined by measuring the voltage applied to the electro-optic crystal 25.

Next, the operation when the object to be measured 13 is largely displaced will be explained using FIG. 3.

FIG. 3(a) shows the relationship between the optical path length difference and the output voltage of the DC differential amplifier 31. Further, FIG. 3(b) shows the relationship between the optical path length difference and the voltage applied to the electro-optic crystal 25. It is assumed that the optical path length difference when the object to be measured 13 is at the origin is XO, and that the operating point is at point a before the device starts operating. When the device operates in this state, a voltage (EoL7) is applied to the electro-optic crystal 25 as shown in FIG. ) move to point b. If the displacement of the object to be measured increases from this state and the optical path length difference reaches X2 through X1'r, the voltage applied to the electro-optic crystal 25 will change as shown in Figure 3 (b). Varies depending on size.

At this time, if the reference voltage kEm of the voltage comparison circuit 19 is set, the output voltage of the voltage comparison circuit 19 is always zero, but becomes 1 when the output voltage of the DC differential power amplifier 31 reaches E+i. Electronic switch 23 via OR circuit 22
As a result, the output voltage of the DC amplifier 32 becomes zero, and the device is balanced at the zero point in FIG. 3(a). When the object to be measured further displaces and the optical path length difference becomes Xa, the balance point similarly moves to point d, and when the optical path length difference becomes X4, voltage B4 is applied to the electro-optic crystal 25 and balance is achieved at point d.

Therefore, the amount of displacement of the object to be measured from the origin position can be determined as follows. The output voltage of the voltage comparison circuit 19 changes from 0 to 1 every time the object to be measured 13 moves by λ/2 (every time the optical path length difference increases by λ), so this is counted as a positive value by the down counter 21. , the result is λ
/2 times the voltage E4 applied to the electro-optic crystal at that time, minus the voltage applied when the object to be measured reaches the origin -Eat, divided by Em, multiplied by 2272, that is, "4" It is good to add Ox'. Suppose that the object to be measured F, rn 2 is displaced from the origin in the opposite direction fc, the reference voltage of the voltage comparator circuit 20 is set to -Em, and the output of the voltage comparator circuit 20 changes from 0 to lVC. When this happens, the amplifier down counter 21 counts negative λ E'4 E
n λ , multiply the result by all times, and then add □×-wo2 Em 2 . (However, E'4 < 0) [Effects of the Invention] As described above, according to the present invention, when a voltage is applied to an optical crystal that adjusts the optical path length, the refractive index changes.
By using fC, the optical system and electrical system are much simpler and the response speed IW is faster, making it possible to measure displacement at tens of thousands of speeds.

[Brief explanation of drawings]

FIG. 1 shows the method for optically measuring displacement according to the present invention.
;i, Rizu, Figure 2 (A) (B) (C) and Figure 3 (
(a) and (b) are explanatory diagrams of the operating principle of the method of the present invention; FIG. 4 is a diagram of the principle of the conventional optical measurement method for displacement;
G41 (1) to (IV) OJ: and Figure 6 (A) (
B) C. It is an explanatory diagram of the operation principle of the conventional method. 4...1/4 wavelength plate, 13...Building side fixed surface. 25... Optical path length control means, 26... Beam splitter cube, 27.28... Polarizing plate, 29.30...
- Photodetector, 31...DC differential amplifier. Agent Tatsuyuki Unuma Figure 1 Figure 5 □ Optical path difference - ◆ Storage capacity (I) Optical path difference - Intensity of 10 million lights (■) Signal at 0 silent operation (A) □ Takako for sending draft 1) Skin pressure (b) -------1 thousand lacquer strength and t negative signal (8) -m-m-meeting U-meeting shaping process → time

Claims (1)

[Claims] (1) A method of measuring the displacement sound of the surface to be measured by irradiating light onto the reference surface and the surface to be measured and using an interference signal corresponding to the optical path length difference of the reflected light obtained by total synthesis of each reflected light. All optical path length control means using a gold element whose refractive index changes when a voltage is applied in the middle of the optical path of the reflected light,
The reflected light that has completely passed through the optical path length control means is incident on the total two-splitting means, and an electrical interference signal with an opposite phase is extracted from the two-split light by the total polarization interference means, and this electrical interference signal is fully amplified to determine the optical path length. Feedback is given to the control means so that the optical path length difference of the reflected light is always constant, and each time the control amount of the optical path length control means exceeds a certain value, the control is returned to zero, and the control amount is set to a constant value. An optical method for measuring displacement, in which the number of times the amount exceeds the specified value is counted, all controlled variables within the certain value are measured, and the total amount of displacement of the object to be measured is measured from the composite value of both.