JPH0478925B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an incident angle measuring method for measuring the incident angle of light onto an object with high precision.

[Prior Art] Conventionally, the angle of incidence Ψ of a luminous flux onto an object is generally measured by determining the position of a spot P on a screen formed by the reflected light reflected from an object 1 and the angle of incidence of the incident light I, as shown in FIG. 5a. This was done by measuring the passing position and calculating the angle between the incident light I and the reflected light. However, this conventional method has the following drawbacks, and it is virtually impossible to measure the angle of incidence with high precision.

[Problems to be Solved by the Invention] First of all, the size of the spot of reflected light cannot be made so small. In order to reduce the size of this reflected light spot, it is necessary to simultaneously reduce the diameter and divergence angle of the incident light beam. However, in principle, the product of these two cannot be reduced below the value determined by light diffraction, so the size of the reflected light spot is the main cause of reducing measurement accuracy in conventional methods. I was getting used to it.

In fact, it is extremely difficult to obtain a measurement accuracy of 1 mrad or more using the conventional method shown in FIG. 5a, and it has been impossible to achieve a measurement accuracy of 0.1 mrad or more. Furthermore, if this method were to be used to improve accuracy, it would be necessary to increase the distance from the object 1 to the measurement position of the reflected light spot, which was a practical obstacle.

Further, FIG. 5b shows a conventional method of measuring an incident angle in which the incident angle Ψ is measured by causing reflected light and transmitted light from the object 1 to interfere with each other. Although this method uses interference and has relatively high measurement accuracy, it has the disadvantage that it interferes with transmitted light and reflected light.
Therefore, there are many difficulties in applying this conventional method as an optical axis adjustment method for other optical measurements. Another major drawback was that it could not be applied to angles of incidence close to normal incidence.

Incidentally, as processing accuracy improves in industrial fields such as semiconductor element manufacturing and precision machining, optical high-precision measurements are increasingly being used. In these fields, it is necessary to make the necessary adjustments for measurement with high precision, so a method that can easily and accurately measure the angle of incidence of light on an object has been desired.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks and to propose a method for measuring the angle of incidence of light on an object with a simple configuration and with high precision.

[Means for solving the problems] In order to achieve such an object, the present invention has the following features:
In a method of measuring the angle of incidence of light on an object,
A diffraction grating is provided on an object, at least one of the diffracted lights by the light diffraction grating is reflected by a reflector to obtain reflected light, and the reflected light is combined with at least one other diffracted light. It is characterized by measuring the angle of incidence of light on an object by measuring the intensity of the light.

[Function] The present invention provides a synthesis method in which a diffraction grating is provided on an object, at least one of the diffracted lights of the incident light by the diffraction grating is reflected by a reflector, and the reflected light and at least one diffracted light are interfered with each other. The angle of incidence Ψ of the incident light on the object is measured by measuring the light intensity of the light U. Since the light intensity of the composite light U is sensitively dependent on the angle of incidence Ψ, the angle of incidence can be determined with high precision. can be measured.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

First, the principle of the present invention will be explained.

Figures 1a and 1b illustrate the principle of the measuring method of the present invention. In the present invention, as shown in the figure, a diffraction grating G is placed on the object 1 in order to measure the incident angle Ψ of the light I incident on the object 1. Generally, from the light I incident on the diffraction grating G, a plurality of diffracted lights D ₁ , D ₂
is obtained.

In Figure 1a, the diffracted light D ₂ is transferred to the reflector 2.
The diffracted light D ₁ and the reflected light R are caused to interfere with each other by being reflected by the diffracted light D 1 to obtain a reflected light R. The reflected light R is caused to travel in the same direction as the other diffracted light D ₁ . The intensity of the combined light U resulting from this interference depends sensitively on the angle of incidence Ψ, as will be described later, so if the intensity is measured by the detector 3, the angle of incidence Ψ can be measured.

At this time, the directions of the reflected light R and the diffracted light _D1 do not need to be exactly the same, but only need to be aligned to the extent that they interfere with each other. Moreover, even if the surface of the reflector 2 is not a perfect mirror surface and has some unevenness,
It is sufficient that the reflected light R can be generated with sufficient intensity to detect the intensity of the combined light U. Furthermore, instead of directly interfering the reflected light R with the diffracted light D ₁ using only one reflector, the reflected light R and the diffracted light D ₁ may be caused to interfere with each other by using a combination of multiple reflectors. . Alternatively, an optical system such as a lens may be interposed between the reflector 2 and the detector 3, thereby causing the reflected light R and the diffracted light _D1 to interfere with each other.

In this way, by using diffracted light, the incident angle can be measured with high precision without having almost any effect on the transmitted light that directly passes through the object 1 or the reflected light that is directly reflected on the object 1. It has the advantage of not interfering with the optical measurement system at all. Also, when using diffracted light for other optical measurements, generally multiple diffracted lights can be obtained from one diffraction grating, so
Select diffracted light that does not affect other optical measurements,
It can be used to measure the angle of incidence.

Here, the object 1 and the reflector 2 do not necessarily have to be different objects, and different surfaces of the same object may be used as the reflector 2. In this case, it is most effective to arrange the reflector 2 parallel to the diffraction grating G, as shown in FIG. 1b. This is because, with the configuration shown in this figure, the direction of the reflected light R can always be made to coincide with the direction of the diffracted light _D1 for any incident angle Ψ.

Further, the parallel arrangement of the diffraction grating G and the reflector 2 is effective for detecting the perpendicularity (Ψ=0) of the incident light I. When the incident light I is close to normal incidence, the second
As shown in the figure, two composite lights U ₁ (combined light of diffracted light D ₁ and reflected light R ₁ ) and U ₂ (combined light of diffracted light D ₂ and reflected light R ₂ ) are obtained symmetrically. . When the incident angle Ψ deviates from the vertical, the composite light U ₁ and the composite light U ₂ exhibit different intensity changes. Therefore, the two combined lights
The point where the intensities of U ₁ and U ₂ are equal is
If the point is detected, it is possible to measure Ψ=0 with extremely high precision and accuracy.

The reason why the incident angle Ψ can be measured by the measuring method of the present invention described above is based on the following principle. That is, since the reflected light R passes through the reflector 2, the phase of the reflected light R is delayed by the length of its optical path. This phase delay Δ is determined by the distance to the reflector 2 and the diffraction angle θ. The diffraction angle θ changes accordingly as the incident angle Ψ changes. Therefore, the reflected light R is the diffracted light D ₁
If the phase delay Δ is measured by the intensity change of the combined light U of the reflected light R and the diffracted light _D1 , the incident angle Ψ can be measured with high precision.

For example, in the example shown in FIG. 1b, the above correlation becomes as shown in the following equation.

First, the diffraction angle θ is given by sinθ=nλ/d+sinΨ (1). However, n is the order of the diffracted light, d is the period of the diffraction grating G, and λ is the wavelength. Here, if the distance between the object 1 and the reflector 2 is S, the phase delay Δ of the reflected light R with respect to the diffracted light D ₁ is Δ=4πS/λcosθ...(2) (However, θ is the phase delay Δ of the diffracted light D ₂ diffraction angle).

Therefore, when the reflected light R interferes with the diffracted light _D1 , the intensity of the combined light _U1 changes according to cos ² Δ, so
By measuring the intensity, the phase delay Δ, that is, the angle of incidence Ψ can be measured according to the above equations (1) and (2).

For example, when using light with a wavelength of 632.8 nm, S=
Even at a short distance of about 100 μm, 4πS/λ takes a value of about 2000, and a measurement accuracy of 10 ^-4 rad can be easily achieved.

By using a composite light of lights I ₁ and I ₂ which are coherent and have different frequencies and polarization states as the incident light, optical heterodyne measurement with a phase delay Δ can be realized with the configuration of the present invention, and the measurement accuracy can be further improved. can be improved. In this case, a clearer beat signal can be obtained by placing a polarizer 4 in front of the detector 3 as shown in FIG. The reason why heterodyne measurement is possible with the extremely simple configuration of the present invention is that the diffraction efficiency of the diffraction grating G and the reflectance at the reflector 2 differ depending on the polarization state of the light.

The amplitude A of the light incident on the detector 3 is the incident light I ₁ ,
Letting the angular frequencies of I ₂ be w ₁ and w ₂ respectively, A=A ₁ (1+αe ^-i 〓)e ^iw1t +A ₂ (-i
Δ)e ^iw2t ……(3). Here, A ₁ and A ₂ are the amplitudes of the diffracted light at the incident light I ₁ and I ₂ components, respectively, and α and β are the diffracted lights of the reflected light R at the incident light I ₁ and I ₂ components, respectively.
It is the amplitude ratio to _D1 . However, in general, α≠β, except when the incident lights I ₁ and I ₂ are both highly symmetrical, such as circularly polarized light. Therefore, the phase φ of the beat of the light intensity detected by the detector 3 is expressed as φ=arctan(α−β)sinΔ/1+αβ+(α+β)c together with the phase delay Δ.
osΔ changes according to (4). Therefore, by measuring the phase φ of the beat of the detected light intensity, the phase delay Δ,
In other words, the incident angle Ψ can be measured.

By using the heterodyne measurement method as described above, measurement accuracy can be significantly improved, and an accuracy of 10 ^-5 rad or more can be easily obtained. Additionally, compared to strength measurement, this measurement method has
It is also extremely superior in that it is almost unaffected by external disturbances such as fluctuations in light source intensity and detector detection sensitivity.

Further, the two component incident lights I ₁ and I ₂ necessary for the above-mentioned heterodyne measurement can be generated by various known methods. For example, electro-optic elements, acousto-optic elements, vibrating mirrors, rotating quarter-wave plates, Zeeman
Using a laser etc., it is easy to separate the two components of incident light I ₁ and I ₂
can be generated.

Moreover, if the method shown in FIG. 3 described above is applied to the symmetrical arrangement as shown in FIG. 2, the accuracy of detecting the verticality of incident light can be significantly improved. In this case, the phases of the beats of the two combined lights U ₁ and U ₂ obtained symmetrically exhibit mutually opposite dependence on the phase delay Δ.
Therefore, the difference between the beat phase φ(U ₁ ) of the composite light U ₁ and the beat phase φ(U ₂ ) of the composite light U ₂ is calculated from the above equation (4) as φ(U ₂ )−φ(U ₁ )=2arctan(α−β)s
inΔ/1+αβ+(α+β)cosΔ...(5) The measurement accuracy improves. Furthermore, this method can easily achieve extremely high measurement accuracy.
This is because when the incident angle Ψ is 0, the phase difference φ(U ₂ )−φ(U ₁ ) becomes 0 from the above equation (5), and detection is easy.

Above, the case where the method of the present invention is applied to the measurement of the incident angle of light has been described, but the method of the present invention
Needless to say, this method can be generally applied to electromagnetic waves and other waves in addition to light.

In fact, as a result of measuring the following example using the measuring method of the present invention, it was possible to measure the incident angle of light with extremely high precision.

Example 1 Using the arrangement shown in Figure 1b, He-Ne laser light (wavelength
The incident angle Ψ of 632.8 nm) I was measured. On the object 1, a diffraction grating G was fabricated by arranging tungsten wires having a thickness of 0.25 μm and a width of 0.2 μm at a pitch of 0.76 μm.
A silicon wafer was placed parallel to the object 1 at a distance of about 60 μm and used as the reflector 2. At this time, the intensity of the composite light U consisting of the ±1st-order diffracted light emitted in the direction of the diffraction angle θ=56° was measured using a photomultiplier tube as the detector 3, and the incident angle Ψ was measured. When the incident angle Ψ is changed, the intensity of the composite light U changes almost as described above, and the intensity changes by 1.
By measuring with an accuracy of 10 -4 rad, we obtained a value of Ψ measurement accuracy of 10 ^-4 rad or more.

Example 2 Furthermore, even higher measurement accuracy was achieved by using a He--Ne transverse Zeeman laser as the incident light source I and performing heterodyne measurement according to the measurement method of the present invention shown in FIG. The Zeeman lasers used at this time have mutually orthogonal polarization directions,
It generates two lights I ₁ and I ₂ with slightly different frequencies, and the frequency, that is, the beat frequency, was 344 kHz. At this time, when the incident angle Ψ is changed using the configuration shown in FIG. 3, the phase (phase difference) of the intensity beat of the composite light U changes linearly with respect to the incident angle Ψ, as shown in FIG. An incident angle/phase sensitivity of 10 ^-4 rad/degree was achieved. Also, the accuracy of phase measurement at this time is approximately 0.2°, therefore,
The measurement accuracy of the incident angle is 2×10 ^-5 rad (approximately 0.1 seconds)
We were able to achieve an extremely high value.

[Effects of the Invention] As described above, according to the present invention, the measurement accuracy of the incident angle can be significantly improved with an extremely simple configuration.

Microfabrication and inspection for semiconductor device manufacturing,
Optical measurement methods are often used for other precision machining, but in order to ensure measurement precision and accuracy, it is necessary to strictly set the incident angle of the light used for measurement. be.
By using the measurement method of the present invention, it is possible to easily measure the angle of incidence with high precision, and to complete the setting of the angle of incidence necessary for optical measurement as described above. Further, since the measuring method of the present invention allows the angle formed by an object with respect to the reference light to be measured with high accuracy, it is also possible to calculate the angle formed by a plurality of objects with high accuracy.

[Brief explanation of the drawing]

Figures 1 a and b and Figures 2 and 3 are diagrams showing examples of the present invention, Figure 4 is a diagram showing measurement results of the method of the present invention, and Figures 5 a and b are conventional examples. FIG. 1...Object, 2...Reflector, 3...Detector, 4
... Polarizer, I ... Incident light, D ₁ , D ₂ ... Diffracted light,
R...Reflected light, _U1 , _U2 ...Synthesized light, G...Diffraction grating, Ψ...Incidence angle, θ...Diffraction angle.

Claims (1)

[Claims] 1. A method for measuring the angle of incidence of light on an object, wherein a diffraction grating is provided on the object, and at least one of the lights diffracted by the diffraction grating is redirected by a reflector. Incident angle measurement, characterized in that the angle of incidence of light on the object is measured by reflecting to obtain reflected light and measuring the intensity of the combined light of the reflected light and at least one other diffracted light. Method. 2. The method of measuring an incident angle according to claim 1, wherein the reflector is arranged parallel to the diffraction grating. 3. The incident angle measuring method according to claim 1, wherein the light is a composite light of mutually coherent light having different frequencies and polarization states.