JP2006030115A - Linear encoder - Google Patents

Abstract

A highly accurate linear encoder is obtained.
Light emitted from a light source 1 is converted into parallel light by a collimator lens 2 and enters an acoustooptic device 3. The acoustooptic device 3 is driven by the fundamental frequency signal output from the oscillator 7, and splits the incident light into 0th-order light E ₀ and 1st-order light E _{+ 1} and emits it to the reflective diffraction grating plate 4. Then, the 0th-order light E ₀ incident from the acoustooptic device 3 is diffracted by the reflective diffraction grating plate 4, and the −first-order light E ₀ ′ is incident on the photodetector 6. Further, the primary light E ₊ 1 incident from the acoustooptic device 3 is diffracted by the reflective diffraction grating plate 4, and the + primary light E ₊₁ ′ enters the photodetector 6. Then, the displacement of the measurement object 100 is calculated from the phase difference between E ₀ ′ and E ₊₁ ′ converted into the beat signal of E ₀ ′ and E ₊₁ ′ detected by the photodetector 6.
[Selection] Figure 1

Description

  The present invention relates to a linear encoder that measures displacement due to parallel movement of an object to be measured.

Laser interferometers, eddy current sensors, capacitance sensors, and the like are known as high-precision linear encoders. These are used for detecting the coordinates of a stage that holds a workpiece in a semiconductor manufacturing apparatus such as a semiconductor exposure apparatus. It is used (for example, Patent Document 1).
JP2003-022948

With the demand for further miniaturization of semiconductor devices, further improvements in accuracy are desired for linear encoders used for detecting the coordinates of a stage holding a workpiece in a semiconductor manufacturing apparatus such as a semiconductor exposure apparatus.
Accordingly, an object of the present invention is to provide a linear encoder capable of measuring displacement due to parallel movement of an object to be measured with higher accuracy.

  To achieve the above object, the present invention provides a linear encoder that measures displacement due to parallel movement of an object to be measured, a diffraction grating that is fixed to the object to be measured, and in which gratings are arranged in the direction in which the displacement is measured, and a light source Branching means for branching the first light emitted from the light source into the second light and the third light having a frequency shift applied to the first light, and emitting the light toward the diffraction grating; A detection means; an nth (where n is an integer) diffracted light of the second light of the diffraction grating; and an mth (where m is an integer different from n) order of the third light of the diffraction grating. Coupling means for coupling the diffracted light of the second light to the light detecting means, the nth order diffracted light of the second light, and the mth order of the third light of the diffraction grating observed by the light detecting means. Phase detection means for detecting the phase of the beat signal due to interference of diffracted light, and detected by the phase detection means Wherein the phase of the serial beat signals to provide a linear encoder that includes a displacement calculating means for calculating the displacement of the object to be measured.

  According to such a linear encoder, the difference in phase change between the n-th order diffracted light and the m-th order diffracted light caused by the displacement in the grating arrangement direction of the diffraction grating caused by the displacement of the object to be measured is calculated. It can be detected by heterodyne detection. Here, since the difference in the phase change depends on the displacement of the object to be measured, accurate displacement measurement can be performed in this way.

Here, in the above linear encoder, the signal of the beat signal is that n is +1 and m is −1, or n is −1 and m is +1. It is preferable for ensuring a large change in the phase of the beat signal with respect to the strength and displacement of the object to be measured.
In the linear encoder described above, the branching unit may include an oscillator that outputs a fundamental frequency signal and an acoustooptic device. In this case, the acoustooptic device is configured to transmit the first light. Zero-order light is emitted as the second light, first-order diffracted light frequency-shifted by the fundamental frequency signal of the first light is emitted as the third light, and the phase detection means includes the oscillator The linear encoder is configured to detect the phase of the beat signal on the basis of the fundamental frequency signal output from the above.

In order to achieve the above object, the present invention provides, as a linear encoder for measuring displacement due to parallel movement of an object to be measured, a diffraction grating fixed to the object to be measured and having a grating arranged in a direction for measuring the displacement. , A light source, branching means for branching the first light emitted from the light source into second light and third light, and emitting the light toward the diffraction grating, light detection means, and the diffraction grating The nth (where n is an integer) diffracted light of the second light and the mth (where m is an integer different from n) diffracted light of the third grating of the diffraction grating are used as the light detection means. A coupling means for coupling;
A phase difference detection means for detecting a phase difference between the nth-order diffracted light of the second light and the mth-order diffracted light of the third light of the diffraction grating, which is observed by the light detection means; There is provided a linear encoder comprising a displacement calculating means for calculating a displacement of the object to be measured from a phase difference detected by a phase detecting means.

  According to such a linear encoder, different phase changes that occur in the n-th order diffracted light and the m-th order diffracted light in accordance with the displacement in the grating arrangement direction of the diffraction grating caused by the displacement of the object to be measured are used. Thus, accurate displacement measurement can be performed.

  As described above, according to the present invention, it is possible to provide a linear encoder that can measure displacement due to parallel movement of an object to be measured with higher accuracy.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 shows the configuration of the linear encoder according to this embodiment.
This linear encoder measures the displacement due to the parallel movement of the measurement object 100 such as a stage for holding a workpiece in the semiconductor exposure apparatus in the left-right direction in FIG.
As shown in the figure, the linear encoder includes a light source 1 such as a laser diode, a collimator lens 2, an acoustooptic device 3, a reflection diffraction grating plate 4, a transmission diffraction grating plate 5, and a photodetector 6 fixed to the object to be measured 100. , An oscillator 7, a phase detector 8, and a displacement calculating device 9.

In such a configuration, the light emitted from the light source 1 is converted into parallel light by the collimator lens 2 and enters the acoustooptic device 3. The acoustooptic device 3 is driven by the fundamental frequency signal output from the oscillator 7, and splits the incident light into 0th-order light E ₀ and 1st-order light E _{+ 1} and emits it to the reflective diffraction grating plate 4. Then, the 0th-order light E ₀ emitted from the acoustooptic device 3 is diffracted by the reflective diffraction grating plate 4, and the −1st-order light E ₀ ′ is incident on the photodetector 6. Further, the primary light E ₊ 1 emitted from the acoustooptic device 3 is diffracted by the reflective diffraction grating plate 4, and the + primary light E _{+ 1} ′ is incident on the photodetector 6. Note that the photodetector 6 uses the 0th-order light E ₀ emitted from the acoustooptic device 3 to be reflected by the reflection type diffraction grating plate 4 −1st-order light E ₀ ′ and the first-order light E ₊₁ emitted from the acoustooptic device 3. The first-order light E ₊₁ ′ of the reflection type diffraction grating plate 4 intersects.

Now, the photodetector 6 detects a beat signal due to interference between incident E ₀ ′ and E ₊₁ ′, and the detected beat signal is phase-detected by the phase detector 8 to obtain its phase. Then, the displacement calculating device 9 calculates the displacement of the measurement object 100 from the phase obtained by the phase detector 8.

Hereinafter, the principle of calculating the displacement of the measurement object 100 in such a linear encoder will be described.
Now, as shown in FIG. 2a, consider a case where an optical plane wave indicated by a vector E _i in Expression 1 is incident perpendicularly to the plate surface of a reflective diffraction grating plate having a grating interval d.

However, E _i0 is the amplitude of the optical plane wave E _i , the vector e ₀ is a unit vector in the polarization direction of light, and ω is the optical angular frequency. Further, the vector k _i is a wave number vector, and the magnitude of the wavelength is expressed by Equation 2, where λ is the wavelength of light. The direction is the traveling direction of the optical plane wave.

In this case, the optical plane wave vector E _{i is} diffracted by the reflection type diffraction grating plate, zero-order light, + first-order light indicated by the vector E _{0 + 1} of the formula 3, shown by vector E _0-1 of the formula 4 - Converted to primary light.

Here, E _{i0 + 1} and E _i0-1 are the amplitudes of ± first-order light, and φ ₀ is a phase change received when the optical plane wave E _i is diffracted. Further, there is a relationship represented by Expression 5 among the vectors k _i , vector k ₊₁ , and vector k ₋₁ . Further, the vector k _i , the vector k ₊₁ , and the vector k ₋₁ are in the same plane.

Here, according to the strengthening condition of the diffraction grating, the phase change of the ± first-order light E _0-1 and E _{0 + 1} with respect to the displacement d in the direction in which the reflective diffraction grating plate engraves the period of the grating is 1 period 2π. Therefore, as shown in FIG. 2b, when the reflection type diffraction grating plate 4 is displaced by x in the direction in which the period of the grating is engraved, the phases of the ± first-order light E _0-1 and E _{0 + 1} are changed by the optical path length Δ by the displacement x. Changes by ± 2πx / d in accordance with the change of Eq.

Here, the phase of the + 1st order light is advanced by 2πx / d and the phase of the −1st order light is delayed by 2πx / d with respect to the displacement of x of the reflective diffraction grating plate. That is, the direction of the phase change between the + 1st order light and the −1st order light is opposite to the displacement of x of the reflective diffraction grating plate, and the phase difference is 4πx / d.
Now, based on the above, the operation of FIG. 1 will be described.
First, light such that light is expressed by the vector of Expression 1 is input from the light source 1 to the acoustooptic device 3. Now, _assuming that the acoustic angular frequency of the fundamental frequency signal input from the oscillator 7 to the acoustooptic device 3 is ω _rf , the zero-order light E ₀ emitted from the acoustooptic device 3 is represented by the vector of Equation 8, The primary light E ₊ 1 emitted from the optical element 3 is expressed as a vector of Expression 9 after receiving the frequency shift of ω _rf by the fundamental frequency signal in the acoustooptic element 3.

However, the wave vector k _i of the zero-order light E _0, the magnitude of the wave vector k _'i of the primary light E ₊₁ is shown by the formula 10 c as light velocity. Further, 0 and the wave number vector k _i of the primary light E _0, and the wave vector k _'i of + 1-order light E _+1, from the conditions of black analysis, as the speed of sound in acousto-optic element 3 v, Equation 11, The relationship indicated by 12 is satisfied.

Next, the minus first-order light E ₀ ′ of the zero-order light E ₀ emitted from the acousto-optic element 3 by the reflection type diffraction grating plate 4 is expressed by Expression 13, and the + first-order light E emitted from the acousto-optic element 3 _{The +} first-order light E ₊₁ ′ generated by the ₊₁ reflection type diffraction grating plate 4 is expressed by Expression 14. Further, 'the wave number vector k _i1 of + 1-order light E _+1' -1 order light E ₀ is the wave vector k _'i1 of, respectively, satisfy the relation of Equation 5.

Then, when the reflection type diffraction grating plate 4 moves x in this direction along with the displacement of the object 100 to be measured, the phase changes as in the case shown in FIG. 4 of the lattice spacing as d, -1 order light E ₀ by the reflection type diffraction grating plate 4 of the 0-order light E ₀ emitted from the acoustooptic element 3 'will change to that represented by the formula 15, an acousto-optic order light E by the reflection type diffraction grating plate 4 of the primary light E ₊₁ emitted from the element 3 ₊₁ 'is changed to those represented by formula 16.

Here, since the magnitudes of the wave number vectors of these two light waves are substantially the same from Equation 10, the transmission type diffraction grating plate 5 having an appropriate grating pitch can be coupled to the photodetector 6.
Now, the light wave E ₀ ″ obtained by diffracting the first-order light E ₀ ′ by the transmission type diffraction grating plate 5 can be expressed by the following equation 17 when the amplitude is set to 1 for the sake of simplicity using an appropriate phase factor φ _1. The light wave E ₊₁ ″ obtained by diffracting the + first-order light E ₊₁ by the transmission type diffraction grating plate 5 can be expressed by Expression 18 when the amplitude is set to 1 for the sake of simplicity using an appropriate phase factor φ _2. .

  Then, a beat signal due to interference between the two light waves is detected by the photodetector 6 and converted into an electric signal V shown in Expression 19.

This electrical signal is phase-detected by the phase detector 8 using the fundamental frequency signal, and the phase of the beat signal is obtained. Further, the displacement x is calculated according to the phase term of Expression 19 from the phase obtained by the phase detector 8 in the displacement calculating device 9.
Here, as the phase detector 8, for example, an I signal is generated by multiplying a fundamental frequency signal and a beat signal, and a Q signal is obtained by multiplying a signal obtained by shifting the phase of the fundamental frequency signal by 90 degrees and the beat signal. A phase detector 8 or the like that generates and determines the phase of the beat signal as a Q / I cotangent can be used. The phase can be obtained in the same manner by using an x signal obtained by multiplying the fundamental frequency signal and the beat signal and a y signal obtained by multiplying the signal obtained by shifting the phase of the fundamental frequency signal and the beat signal by 90 degrees.

  In addition, the displacement calculation device 9 registers Φ1 and Φ2 obtained in advance with respect to the reference position of the measurement object 100 by calculation or experimentally, and the displacement calculation device 9 uses the Φ1 and Φ2. The displacement x is calculated. It is also possible to calculate the change of the displacement x, that is, the speed of the measurement object 100 from the change of the phase, or to calculate the integral of this speed as the displacement of the measurement object 100.

The embodiment of the present invention has been described above.
In the above, the reflection type diffraction grating plate 4 is fixed to the object to be measured 100, but this is a transmission type diffraction grating instead of the reflection type diffraction grating plate 4 depending on the object to which the linear encoder is applied. It is also possible to fix the plate to the object to be measured 100 for use. Further, in the above, the displacement of the object to be measured is caused by interference between the + first-order light of one of the two lights branched by the acoustooptic device 3 and the -1st-order light of the other light. Of the two light beams branched by the acousto-optic device 3, the n-order light (n is an arbitrary integer) by the reflective diffraction grating plate 4 and the other light m (m is Arbitrary integers, m ≠ n) The displacement of the object to be measured can be measured from the interference of the next light.

  As described above, according to the present embodiment, the displacement of the diffraction grating fixed to the object 100 to be measured is measured by heterodyne detection of the change in phase that occurs in the diffracted light due to the displacement. Therefore, it is possible to measure the displacement with high accuracy.

It is a block diagram which shows the structure of the linear encoder which concerns on embodiment of this invention. It is a figure which shows the phase change by the shift of the reflection type | mold analytical grid plate in the linear encoder which concerns on embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Collimator lens, 3 ... Acoustooptic element, 4 ... Reflection type diffraction grating plate, 5 ... Transmission type diffraction grating plate, 6 ... Photo detector, 7 ... Oscillator, 8 ... Phase detector, 9 ... Displacement calculation apparatus 100 to-be-measured object.

Claims (4)

A linear encoder that measures displacement due to parallel movement of an object to be measured,
A diffraction grating fixed to the object to be measured and arranged in a direction to measure displacement;
A light source;
Branching means for branching the first light emitted from the light source into the second light and the third light obtained by giving a frequency shift to the first light, and emitting the light toward the diffraction grating;
Light detection means;
The nth (where n is an integer) diffracted light of the second light of the diffraction grating and the mth (where m is an integer different from n) diffracted light of the third light of the diffraction grating. Coupling means coupled to the light detection means;
Phase detection means for detecting a phase of a beat signal caused by interference between the nth-order diffracted light of the second light and the mth-order diffracted light of the third light of the diffraction grating, which is observed by the light detection means When,
A linear encoder comprising: a displacement calculating means for calculating a displacement of the object to be measured from the phase of the beat signal detected by the phase detecting means.
The linear encoder according to claim 1,
The linear encoder characterized in that n is +1 and m is -1, or n is -1 and m is +1.
A linear encoder according to claim 1 or 2, wherein
The branching means includes
Including an oscillator that outputs a fundamental frequency signal and an acousto-optic element;
The acoustooptic device emits the 0th-order light of the first light as the second light, and the first-order diffracted light frequency-shifted by the fundamental frequency signal of the first light is the third light. Emanating as
The linear encoder characterized in that the phase detection means detects the phase of the beat signal with reference to the fundamental frequency signal output from the oscillator.
A linear encoder that measures displacement due to parallel movement of an object to be measured,
A diffraction grating fixed to the object to be measured and arranged in a direction to measure displacement;
A light source;
Branching means for branching the first light emitted from the light source into the second light and the third light and emitting the light toward the diffraction grating;
Light detection means;
The nth (where n is an integer) diffracted light of the second light of the diffraction grating and the mth (where m is an integer different from n) diffracted light of the third light of the diffraction grating. Coupling means coupled to the light detection means;
A phase difference detection means for detecting a phase difference between the nth order diffracted light of the second light and the mth order diffracted light of the third light of the diffraction grating, which is observed by the light detection means;
A linear encoder comprising: a displacement calculating unit that calculates a displacement of the object to be measured from a phase difference detected by the phase detecting unit.