JPS6337202A - Optical apparatus for measuring minute displacement - Google Patents

Abstract

PURPOSE:To enable the measurement of minute displacement by means of a light source which facilitates the adjustment of measurement and is easy to handle, by forming an interferogram by a two-beam interferometer by using a white light source, and by measuring the displacement of the center burst thereof by using an image sensor. CONSTITUTION:A flux of light emitted from a white light source 1 to a Michelson interferometer 20 and reflected by a fixed reflector 26 inclined by a minute angle theta and by a moving reflector 25 fitted perpendicularly to a moving body 24 forms an interferogram on an image surface. An image sensor 30 is provided on the image surface of this interferometer 20, and when a center burst before displacement is positioned at an i-th photoelectric conversion element of a sensor 30, first, this is determined by a discriminating means 41. Next, a switch SW1 is switched over, and a discriminating means 42 determines that the center burst after displacement is positioned at a j-th photoelectric conversion element. Based on these determinations, a computing means 43 takes-in the pitches P and angles theta of the photoelectric conversion elements and calculates tantheta(i-j)P. This absolute value represents an amount of the displacement of the moving body 24.

Description

[Detailed description of the invention] Industrial use! '? ] The present invention provides a positioning mechanism for an X-Y stage of a microscope, etc.
The present invention relates to an optical minute displacement measuring device used in a stepper of semiconductor manufacturing equipment, a positioning mechanism for a moving mirror of a Fourier spectrometer, and the like.

[Prior art J As one of optical minute displacement measurement techniques, He-Ne
An interferometer with a monochromatic light source such as a laser is used to form sine wave interference fringes on the image plane, and the output signal of the image sensor placed on the image plane is Fourier transformed to calculate the phase term. A technique for determining Al1 the displacement of a moving mirror based on a phase term is known.

[Problems to be Solved] However, according to the "-2 measurement technique, there are the following problems.

In order to create sinusoidal interference fringes on the CO image plane, t
It is necessary to use a monochromatic light source such as a le-Ne laser, which complicates the device configuration and increases costs.

(2) Furthermore, the algorithm for calculating the target displacement 14 from the phase term after Fourier transformation is complicated and takes a long calculation time. Therefore, restrictions are placed on the purpose of real-time processing.

(2) If the method of determining the amount of displacement from the phase spectrum is adopted, it becomes possible to measure minute displacements on the order of 1/100 of the wavelength or less, but conventionally only a single wavelength element is used. , 1111 fixed precision 1■ lacks reliability.

The present invention solves the problem mentioned above, and aims to provide an optical minute displacement measurement bag that allows easy measurement adjustment and uses a simple light source.

Means for Solving Problem E] In order to solve the above problem, the optical minute displacement measuring device according to the first invention of the present application consists of the following five constituent elements.

■There must be a white light source.

The term "white light source" refers to a monochromatic light source, and may be a light source having a continuous spectrum in a specific wavelength range, for example.

(2) There is a two-beam interferometer that receives the white light and spatially creates an interferogram on the image plane that has an interferogram centered on the optical path difference as a variable.

(2) There is an image sensor having a large number of photoelectric conversion units in the transverse direction of the interferogram.

(2) There is a means for determining the order of the photoelectric conversion units that output a peak value among the photoelectric conversion units of the image sensor before and after the optical element that provides the optical path difference is displaced.

(5) Calculate the difference in the order of both photoelectric conversion units, calculate the displacement of the center burst based on the difference, and calculate the product of the displacement of the center burst and the tangent of the inclination of the reflecting mirror that provides the optical path difference. There must be a calculation means for calculation.

The optical micro-displacement measuring device according to the second invention of the present application consists of the following six constituent elements.

■There must be a white light source.

There is a two-beam interferometer that receives white light and spatially creates an interferogram with a center burst on the image plane, with the optical path difference as a variable.

(2) There is an image sensor having a large number of photoelectric conversion units in the transverse direction of the interferogram.

(2) There is a means for collecting white light interferograms before and after displacement of the optical element giving the optical path difference.

■There must be a means to Fourier transform these interferograms.

■ Using the real part term and imaginary part term of each Fourier transform result, find the phase of each wave number element 41f:, calculate the displacement amount for each wave number element from the difference in the phase spectrum before and after the displacement, There must be a calculation means for statistically processing them to obtain the final amount of displacement.

"Example] Next, embodiments of the present invention will be described based on the drawings.

FIG. 1 is a block diagram showing an embodiment of an optical minute displacement measuring device according to the first invention of the present application.

1 is a white light source, such as a small light bulb or a light emitting diode, which has a continuous spectrum in a specific wavelength range. 20 is a Michelson interferometer in which a fixed reflecting mirror 26 is tilted by a small angle of 0, and includes an aperture 21 that limits the solid angle of reception with respect to the white light source 1, a collimator lens 22 that converts the incident light beam into a parallel light beam, and a collimator lens 22 that converts the incident light beam into a parallel light beam. A beam splitter 23 that divides the amplitude into two beams, and a beam splitter 23 that divides the amplitude into two beams, and
111 A movable reflector 25 placed perpendicularly to the movable body 24 to be fixed, and a fixed reflector 26 provided with an inclination angle of 0 to the vertical direction of the optical axis in the other optical path, fixed by both light beams. It is composed of an imaging lens 27 that forms an image of interference fringes localized in the vicinity of a reflecting mirror 26.

Incidentally, the light beam reflected by the moving reflector 25 and the light beam reflected by the fixed reflector 28 are superimposed by the beam splitter 23, and an interferogram (intensity pattern of interference light) with the optical path difference as a variable is spatially formed on the image plane. is created, but
When the light source is monochromatic light, the interference fringe pattern is like a sine curve, but when the light source includes various wavelengths as in this example, the interference fringe pattern is like a sine curve for various wavelengths. As a result, as shown in FIG. 2, the interferogram becomes a pattern of interference fringes having a center burst (peak value) with zero optical path difference and vibration damping on the left and right sides of the center burst (peak value).

Now, the movable body 25, that is, the movable reflecting mirror 25 is displaced to the right direction f1. Suppose that it is displaced by d. If the position of the center burst before displacement is Ll, and the position of the center burst after displacement is L2, the center burst is displaced to the right by 1L=L2-Ll. Therefore, by measuring the direction of displacement of the center east, the direction of displacement of the movable reflecting mirror 25 can be identified. Since this is the case where the optical path difference is zero at the center burst position, the following equation holds between the displacement amount d of the movable reflecting mirror 25 and the displacement amount of the center burst.

d-tanO・L...(1)
Here, by measuring the displacement scale of the center burst as a landmark, the displacement amount d of the movable reflecting mirror 25 can be determined by the product of the tangent of the known inclination angle 0 and the displacement scale. When the inclination angle 0 is finely adjusted, the movable reflector 25
The measurement range of the displacement d can be varied, and the inclination angle of 0 is significant as a scale factor. Therefore, displacement measurement in a wide range of measurement areas is possible.

Next, on the image plane of the interferometer 20, there is an image sensor 30 having a large number of photoelectric conversion units in the transverse direction of the interferogram.
is provided. The image sensor 30 sequentially scans its photoelectric conversion section at predetermined intervals and outputs the intensity distribution of the interferogram as a serial signal, and uses, for example, a self-scanning photodiode array. 31 is a preamplifier that amplifies the output signal of the image sensor 30. 32 is an analog/digital converter that converts the amplified analog signal of the interferogram into a digital signal and outputs the digital signal.

A signal processing section 40 processes the digitalized interferogram data and is used to calculate the displacement of the movable reflecting mirror 25. The functions of this signal processing section 40 are shown as blocks within broken lines.

First, the interferogram before displacement has the intensity distribution shown in FIG. The previous peak value determining means 41 determines this i-th photoelectric conversion unit. Specifically, the intensity signal of the interferogram before displacement is transmitted to the image sensor 30.
The center burst is stored once in the address number corresponding to the order of the photoelectric conversion units, and the address number (=ii-th) having the largest content among each address number is stored.Next, the center burst position after displacement is set to the left. If it is in the photoelectric conversion unit number j [1], the swift SWI is terminated, and the peak value discrimination means 42 discriminates the photoelectric conversion unit number 1 and 11.
This determination of the photoelectric conversion unit can be performed in a time-sharing manner.
The peak value 'rIl of the funeral can be realized in a separate step. The calculation means 43 calculates the difference between number i and number 1 and 11, and calculates this difference (i-j)
Based on the pitch P of the photoelectric conversion section of the image sensor 30 manually inputted from the outside, the displacement of the center berth (
i-J) Calculate P and calculate t from the inclination angle 0 input from the outside.
an(7, then tan O(i j
) P is calculated and outputted to the output device 44. (i
−j) indicates a rightward displacement, and a negative value indicates a leftward displacement, and its absolute value is the amount of displacement (amount of movement)
It represents. However, here, the magnification of the imaging lens 27 is assumed to be 1 for simplicity, but generally the magnification is set to k.
In this case, the result of the two-note operation can be multiplied by k.

FIG. 3 shows the optical minute displacement Jllll according to the second invention of the present application.
It is a block diagram showing one example of a fixed neck. This embodiment measures displacement with high precision without measuring the inclination angle O. In FIG. 3, the same parts as those shown in FIG. 1 are designated by the same reference numerals, and their explanations will be omitted.

First, to explain the measurement principle, let us assume that the wave number of one monochromatic light included in the white light source 1 is 6 degrees. is G (x) = b (d.) cos (2πf, x
+ 5' Cd, ) ) + a
...(2) Here, a; DC bias component, b(σ)
; Spectrum of the white light source 1, fo; Spatial frequency of interference fringes, Cf (((, ); Phase term corresponding to wave number d.;
) Set the center to 0.

Further, the following equation holds true between the spatial frequency f of the interference fringe and the inclination angle θ.

fo = 2J, tan O-(3) Therefore, by substituting equation (3) into equation (2), G (x)
= b (6o) cos (4qJ, tan O
a x+p)) + a...(4) Therefore, the intensity L of the interference fringe for all wave numbers of the white light source l
(x), that is, the interferogram, by superposition, L (X) = f [b (, [) cos
(4π(:jtan O◆x+ CfC(f)l
+ a] #' ・(5) This AC term is I (x)
Then, I (x) = fb (go) cos (4π
dtan Oe x+f(g))d...
(B) If b(d) is divided into even and odd function components, b
((j)-be((I5')+b,(d) ・-
(7) Substituting equation (7) into equation (6) and organizing it, I
(x) =, L b, (d) exp (j Cf
(d) IXexp (j 4πtan Oe x
cfl d4j= 1=
...(8) Here, be(J) is a new white light source l
Let the spectrum B(6) be 4πtanO*x=X, then I T (X) = f B (1) exp (j
(-fcO”) IX exp (j Xd)
dl - (9) From equation (8), B (
d) E! Since we know that IP (j fC and)) is given by the inverse Fourier transform of i (X), here, C(6') = f I (X) cos (
X)dX-2S Cd) = f I (X) sin
(X)dX, equation (10) becomes B Cd ) cas(fc(f) + j B (d
) sintf(J)=C(J)−jS(and)
...(11) Yotte, tany) (J) -
−S Cd ) / CCtj ) 1!0, phase term f
(go) is ヅ(d) -tan (-3(gu) /C(J)
) (12) By the way, the following equation holds true between the displacement amount d and the phase term (-f(g)).

'-f(go)=4πdgo...(13
) From equations (12) and (13), d(d) should be determined, but due to reasons such as variations in sensitivity of the photoelectric conversion part of the image sensor 3o, it cannot be determined uniquely for all wave numbers d. Must not be. There, the amount of displacement d- is determined for each wavenumber element.

If we consider that the white light source l consists of N wavenumber elements and has uniform intensity, then for each displacement amount,
By taking the arithmetic mean of the SIN ratio (standard deviation)
will be improved in proportion to F'. As a precise statistical process, when considering the spectral characteristics of the white light source l, LT
; Obtain the power spectrum given by 7, take into consideration the intensity distribution of each wavenumber element, and take a weighted average.

To explain the measuring device, the signal processing section 5 shown in FIG.
0 collection means 51 collects the interferogram before displacement. The collecting means 51 is a type of storage device, and has the number of addresses corresponding to the number of photoelectric conversion units of the image sensor 30. A frequency analyzer 53 as a Fourier transform means performs a Fourier transform on the interferogram before displacement, and an arithmetic unit 55 uses the real part term and imaginary part term of the result of the Fourier transform to obtain the result shown in FIG. A power spectrum (amplitude spectrum) AI as shown in ) and a phase spectrum P1 as shown in FIG. 4(B) are obtained. Now, since the reference point is set at the center of the image sensor 30, the phase spectrum PI is 0 between the measurement areas P to 4. However, the position of the reference point may be arbitrary. Next, after the displacement, the switch SW2 is switched, the collecting means 52 collects the interferogram after the displacement, the frequency analyzer 54 performs Fourier transform on this, and the arithmetic unit 55 calculates the real part of the Fourier transform result. Using the term and the imaginary part term, the fourth
A power spectrum A2 as shown in FIG. 4(A) and a phase spectrum P2 as shown in FIG. 4(B) are obtained.

Since the power spectrum depends on the spectral characteristics of the white light source,
Power spectra A1 and A2 overlap. The collection and Fourier transformation of such interferograms are as follows:
Since it can be performed in time division, it can be implemented with a single acquisition means and frequency analyzer. Next, the arithmetic unit 55 calculates the displacement amount of each wavenumber element (σ) from the difference between the phase spectra before and after the displacement, statistically processes them, and supplies the final displacement amount to the output device 58. Since the difference between the phase spectrum phase P1 and the spectrum P2 after displacement is the phase term (fcd) associated with displacement, using equation (13), d=Lf(0/)/4πd, the power spectrum A1 and A weighted average is calculated taking A2 into consideration.

According to this device, under the best conditions, the S/N ratio is improved in proportion to the square root of the number of wavenumber elements and the number of photoelectric conversion units of the image sensor 30, and a highly accurate displacement on the order of 171 OO or less of the wavelength is achieved. It becomes possible to

In calculating the displacement d, all wavenumber elements emitted from the white light source 1 are used, so a fast Fourier transform (FFT) algorithm is used for the Fourier transform operation η. However, when the number of wavenumber elements is limited to a few, it is desirable to use #l: Dispersive Norier Transform (DFT) from the viewpoint of calculation time.

[Effects of the Invention] As explained above, the optical minute displacement measuring device according to the first invention of the present application is characterized in that the displacement is measured using the interburst of the interferogram as a landmark. . Since the position before and after displacement of the interburst can be easily measured, measurement adjustment is easier than when measuring the displacement of interference fringes caused by monochromatic light.
Moreover, since a simple light source can be used to create an interferogram, the device configuration can be simplified without being restricted by the light source. Further, by varying the inclination angle θ, there is an effect that the measurement area can be widened. By using an image sensor, interferograms can be instantaneously processed as electrical signals, making it possible to measure displacement in almost real time, making it easier to control moving objects, and controlling X-Y signals such as microscopes. Stage positioning mechanism.

It can be easily applied to steppers in semiconductor manufacturing equipment, positioning mechanisms for moving mirrors in Fourier spectrometers, and the like.

The optical minute displacement ^III device according to the present FtEJ2 invention measures displacement from the phase of various wavenumber elements σ, and the amount of displacement can be calculated by the number of wavenumber elements, and uniform λ1 processing is performed in the cylinder. This allows the displacement amount to be determined with high accuracy. Further, similar to the first invention of the present application, a simple light source for creating an interferogram can be used instead of a monochromatic light source such as a He-Ne laser.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an optical minute displacement measuring device according to the first invention of the present application. FIGS. 2A and 2B are waveform diagrams showing an interferogram before displacement and an interferogram after displacement in the same embodiment. FIG. 3 is a block diagram showing an embodiment of the optical minute displacement measuring device according to the second invention of the present application. FIGS. 4(A) and 4(B) are graphs showing the power spectrum and phase spectrum of the interferogram in the same example. 1... White light source, 20... Michelson interferometer with fixed reflecting mirror tilted, θ, φ, tilt angle of fixed reflecting mirror 2B, 25
・War φ moving reflector, 28...Fixed reflector, 30111
10 image sensor-131...preamplifier, 32.
...Analog/digital converter, 40.50...Signal processing unit, 41.42...Fist discrimination means, 43.55...
- Arithmetic means, 44, 58... Output device, 51.52
...Collection means, 53.54...ψ frequency analyzer.

Claims (2)

[Claims] (1) A white light source, a two-beam interferometer that receives the white light and spatially creates an interferogram on the image plane having a center burst with an optical path difference as a variable, and an image sensor having a photoelectric conversion section; a means for determining the order of the photoelectric conversion sections that output a peak value among the photoelectric conversion sections of the image sensor before and after displacement of the optical element that imparts the optical path difference; Calculating means that calculates the difference in the order of the photoelectric conversion units, determines the displacement of the center burst based on the difference, and calculates the product of the displacement of the center burst and the tangent of the inclination of the reflecting mirror that provides the optical path difference. An optical minute displacement measuring device comprising: (2) A white light source, a two-beam interferometer that receives the white light and spatially creates an interferogram on the image plane having a center burst with an optical path difference as a variable, and a large number of beams in the transverse direction of the interferogram. An image sensor having a photoelectric conversion section, means for collecting white light interferograms before and after displacement of the optical element imparting the optical path difference, means for Fourier transforming these interferograms, and respective Fourier transforms. Using the real part term and imaginary part term of the result, find the phase for each wavenumber element, calculate the displacement amount for each wavenumber element from the difference in the phase spectrum before and after the displacement, and statistically process them. 1. An optical micro-displacement measuring device comprising: arithmetic means for determining the final amount of displacement.