JPS6024401B2 - How to measure the physical constants of a measured object - Google Patents

Abstract

PURPOSE:Wave faces with phase gap illuminated by the light beam with thickness of wave length are divided. Each divided face is inclined and folded to measure such physical factors as thickness and reflection factor without using a vibration mirror.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a film, an emulsion thickness coated on the film, an I
Generally speaking, physical constants such as the film deposited on a substrate or glass, the distance between glasses, the thickness of an optically transparent substance, or the refractive index of the film, etc. This relates to a non-destructive, non-contact measurement method.

Conventional methods for measuring the above-mentioned physical constants include adding a step to the object being measured in some way and measuring the film thickness with a stylus, and another method of adding a step and measuring the film thickness using an interference microscope. , measure the spectral reflectance from the object to be measured,
Methods of detecting concentration by calculating from the wavelengths at which the maximum and minimum reflections occur are known, but the first two methods require a step to be placed on the object to be measured, in other words, it is necessary to break the soil. When using a stylus, it tends to stick to the object to be measured, and when using the latter, it takes time to measure, and the film thickness is measured by detecting the maximum and minimum wavelengths, so it is less accurate. Each had their own drawbacks.
A method for carrying out the measurement in a non-destructive and non-contact manner is disclosed in Patent Application Publication No. 12192 of 1960. In this method, light from the first and second surfaces of the object illuminated by a light source with a wavelength width is guided to an interferometer such as a Michelson interferometer, and one of the divided optical paths of the interferometer is guided. The optical path length is set over a certain span, that is, one of the two reflecting mirrors to which the two wavefronts split by the light splitter of the Michelson interferometer are directed is set over a certain span such that an optical path difference occurs with respect to the other. The amount of movement during this vibration is expressed on the time axis, and the elapsed time between the time when the interference peak caused by this vibration appears and the time corresponding to a certain position of the vibrating mirror in the span. measure,
This method measures the physical constant as a function of its elapsed time. Since this method is characterized by measuring elapsed time, the oscillating mirror must be direct with respect to time. It is very difficult to obtain a beam that vibrates directly over such a period of time. This is clear from the fact that the inventor of this method does not measure the elapsed time in the specification, but instead describes a method of measuring the number of pulses corresponding to the amount of movement of the vibrating mirror. The main object of the present invention is to provide a measurement method that produces an optical path difference without using such a vibrating mirror.

The method of the present invention will be explained below.

1, 2, and 3 explain the principle of the method of the present invention. In FIG. 1, a light source 2 from a white light source 1 having a wavelength width irradiates an object 3 to be measured.

Here, the light beam is reflected by the first reflecting surface of the object to be measured 3, becomes the light beam 5, and is refracted into the object to be measured, and is reflected by the second reflecting surface, which becomes the light beam 4. The east light is refracted from the object to be measured and is emitted to the outside, becoming a light east 6 parallel to the reflected light east 5. now,
Assuming that the thickness of the object to be measured is d, the refractive index is n, and the angle of incidence of the light beam on the second reflective surface is 0, an optical path difference of d cos 0 occurs between the light beam 5 and the light beam 6. This means that These two light beams have a beam splitter 7, mirrors 9 and 11,
It enters a so-called Michelson type interferometer consisting of a lens 12, where it is amplitude-split by a beam splitter 7 to become Koto 8 and Koto 9. Koto 8 is reflected by mirror 9 and is beam split again. The light passes through the lens 12 and enters the screen 13 through the lens 12. The other light beam 10 is reflected by a mirror 11, reflected again by a beam splitter 7, and enters a screen 13 via a lens 12. Now, consider a case where the mirror 11 is relatively larger than the mirror 9. FIG. 2 shows a virtual image 11' of the beam splitter of mirror 11.
This is a diagram depicting the mirror 9 itself 9'. In this figure, each mirror is relatively crowded. Further, the light beam 5' enters the mirror 9' perpendicularly. If two light beams 5' and 6' with a phase difference are incident on this mirror system, the component contributing to interference is the mirror 11' of light beam 6'.
The white interference fringe is localized at 14, where the two intersect. If we take the x-axis parallel to mirror 9', the localized positions of these white interference fringes can be approximated by mirror 9' and mirror 11'.
At a point ndcos g/tan distance from the intersection point 15, x = Mcos, and /neno. In the vicinity of the intersection 15 of these white interference fringes, white interference fringes are generated by each of Koto 5' and Koto 6', and regarding the intersection 15, the opposite direction x=-ndcos0nonenn is also produced by Koto 5. ' and Koto 6' produce white interference fringes. The lens 12 in FIG. 1 images these localized white interference fringes onto the screen 13. When the imaging magnification of the lens 12 is equal to the same magnification, white interference fringes projected onto the screen 13 are observed as shown in FIG. At this time, the light intensity distribution in the x-axis direction is 1(X)=Ji(k)Cos2(kndCos○
) COS2(kxnen8)dk. However, i (
k) is given by multiplying the spectral distribution of the light source, the spectral sensitivity of the observation system, and the spectral transmittance of the optical system. Further, k is a wave number. FIG. 4 depicts the general shape of this light intensity distribution. Now, the white interference peak that occurs at x=0 is the central peak, the white interference peak that occurs at x=Mcos/Nen is the first side peak, and x=-ndcos0/
If the white interference peak produced by tano' is the peak on the second side, then from where the central peak occurs to where the peak on the first side or the peak on the second side occurs,
Or from where the peak on the first side occurs to where the peak on the second side occurs, or from any predetermined reference point on the x-axis to where the peak on the first side or the peak on the second side occurs. By detecting the interval up to the point, ndcosJ/tano can be obtained.

Therefore, by substituting the refractive index n of the object to be measured, it is possible to measure the thickness d of the object to be measured. If the refractive index n of the object to be measured is also unknown, the refractive index n and thickness d can be determined by measuring the irradiation angle ◇ to the object to be measured twice, for example, 0=00 and 0=45o. It is possible to obtain each independently.

In Figure 1, the object to be measured has two reflecting surfaces. However, in the case of an object having two or more reflecting surfaces, that is, in the case of an object consisting of multiple layers, it is possible to When there is sufficient reflected light, white interference fringes are obtained at locations corresponding to the thickness of each layer, and it is also possible to measure the thickness of each layer. Also, in birefringent monsters,
The retardation that occurs between the p-polarized light beam and the s-polarized light beam transmitted therethrough can also be measured in the same manner as described above. In the method according to the present invention, the mirror of the interferometer only needs to be fixed at a relatively slight angle.
Unlike the method of No. 1-12,192, this method has the great advantage that there are no moving parts such as moving the mirror in the interferometer, which has to be adjusted very delicately.

Other advantages will become apparent in connection with the method for detecting spatially generated white interference fringes, described below. One way to detect white interference fringes is to place a one-dimensional photodiode array in the screen section of FIG. 3 parallel to the x-axis. By sequentially scanning the photodiode array in this manner, white interference fringes converted to the time axis are obtained, and the thickness of the object to be measured can be determined from the time interval between interference peaks. The use of this photodiode array means that
In the method of 12192, it is extremely difficult to obtain a movable mirror with a linear movement amount to change one of the divided optical paths of the interferometer, whereas a photodiode array has extremely good linearity. Take advantage of this advantage. In fact, this type of device in which a large number of photodiodes are arranged in one dimension (several hundred to 2,000) is commercially available from Reticon, Inc. in the United States, and can be used immediately in carrying out the present invention. To detect this white interference fringe, an I
A TV can also be used. As another example, as shown in FIG. 5, a rotating mirror 16 is rotated or vibrated around a fulcrum 17 as shown in 18, and white interference fringes are detected using a pinhole 19 and a photodetector 20. You may do so. At this time, considering that the pinhole 19 is formed one-dimensionally as shown in FIG. 3, if a slit is used, a larger amount of light can be obtained. In addition, as another example, the sixth
As shown in the figure, the pinhole or slit 21 may be moved in the direction of the arrow 23 and the distribution of white interference fringes may be detected by the photodetector 22.

The scanning system that moves the pinhole or slit in this way reads the formed white interference fringes.
Compared to the vibrating mirror described in No. 2192, it does not need to be as precise.

Therefore, this embodiment does not use a vibrating mirror that requires precision on the wavelength order to form white interference fringes, and has the effect that the formed white interference fringes can be observed with a simple scanning slit or pinhole. are doing. In addition, in any of the above methods for detecting white interference fringes, a larger amount of light can be emitted by further aligning a cylindrical lens with the lens 12, taking into consideration that white interference fringes are formed one-dimensionally. .

FIG. 7 shows an optical system using this cylindrical lens. The light emitted from the light source 101 is transferred to the imaging lens 102
The light beam is focused on the surface of the object to be measured 103, and the reflected light from the object to be measured is turned into substantially parallel light by the lens 104, and guided to a tilt-type interferometer 105. White interference fringes localized near the tilt mirror of the interferometer 105 are imaged onto a -dimensional photodiode 108 by anamorphic optical systems 106 and 107. At this time, the focal length of the cylindrical lens 107, which has an imaging function in the vertical direction, is made shorter than the focal length of the cylindrical lens 106, which has an imaging function in the direction in which the diodes are arranged, as shown in FIG. The vertical magnification of the white interference fringes can be reduced compared to the horizontal magnification, and the amount of light of the white interference fringes can be effectively guided to the one-dimensional photodiode surface. As for the interferometer, a Michelson type interferometer is taken as an example, but it is also possible to use a Machender type interferometer or the like in implementation. Further, optical systems similar to these interferometers using a Wollaston prism, Rochon prism, or Sinalmon prism can also be used. An example using a Wollaston prism is shown in FIG. The Wollaston prism is made by cutting out two birefringent materials, such as crystal, with optical axes in the 54 and 55 directions and pasting them together. 53 is a polarizer, and its polarization direction is adjusted to be 45 degrees with respect to the optical axis of crystals 54 and 55.

Further, 56 is an analyzer, which is arranged so as to be in parallel or crossed Nicols with the polarizer. In this system, there is a wavefront 51 of the light beam 5 reflected from the surface of the object to be measured 3, and a wavefront 51 of the light beam 5 from the back surface whose optical path difference is delayed by 2 bcos.
When the wavefront 52 is incident, the wavefronts 51 and 52 are divided into two by the Wollaston prism, and a square is created in the wavefront, resulting in wavefronts 51' and 52' and wavefronts 5r and 52. Now, if we take the y-axis as 62 with the point where the thickness of prism 54 and prism 55 are equal as the origin, the wavefront 51
The optical path difference that occurs between ' and the wavefront 5r is such that the refractive index for the ordinary ray of birefringence is no, and the refractive index for the extraordinary ray is ne.
Then, 2 (fertile blood) is given by Xan8. wave front 5
1' and the wave surface 51'', and the wave surface 52' and the wave surface 52'' interfere near 58, that is, near y=0, and form a central peak. The wave surface 51' and the wave surface 52' interfere in the vicinity of 57, and the wave surface 52' and the wave surface 51'' interfere in the vicinity of 59, which are the peaks of the respective sides.This is approximately the fox dcos0. and 2(M-blood)×a
where n is equal, that is, y=ndcosma/(n
e-blood) occurs near the tan. These white interference fringes are localized near the Wallaston prism, and the lens 6
0, these white interference fringes are imaged on a photodetector 61 such as a photodiode array. In this way, it is possible to measure the thickness and refractive index of the object to be measured using the Wollaston prism in the same way as the Michelson interferometer or the like. If the thickness of the object to be measured is at most a few meters, the wavefront width may be approximately several minutes in angle. If the Wollaston prism were to be made of quartz crystal, the thickness of the Wollaston prism would be about two sides in order to give this degree of wave front head, and an extremely compact sensitometer could be constructed.

In this way, compared to the Michelson interferometer, an interferometer using a Wollaston prism can determine the inclination of the wavefront at the time of making the prism, and unlike the Michelson interferometer, it is not necessary to adjust the mirror inclination at the time of assembly. It has many advantages, such as not having any problems, not changing over time, and being extremely compact as mentioned above. In the embodiment shown in FIG. 7, an optical system was explained in which when a one-dimensional photodiode array is used, the white interference fringes are reduced in the direction perpendicular to the arrangement direction of the array and the amount of light is effectively used. The anamorphic optical system used in this optical system generally has a large amount of aberration.

Next, FIGS. 9 and 10 a show examples of other optical systems that effectively guide the amount of interference fringe light to a one-dimensional photodiode array.
, b.

FIG. 10 is a developed view of the optical system, in which a is a cross-sectional view and b is a view seen from above. The light from the white light source 110 is converted into parallel light by the lens 111, and the diameter of the light is compressed in the vertical direction by the afocal cylindrical lens systems 112 and 113, thereby illuminating the slit Sekiguchi 114. The parallel light passing through the slit dark opening 114 passes through a polarizing plate 116 and is focused onto the object surface 117 by a lens 116. In the present invention, it is not necessarily necessary to focus the light on the surface of the object to be measured, but if the surface shape of the object is poor, focusing the light can reduce the influence of the surface shape and improve the output signal from the diode array. The SN ratio becomes good.

The reflected light from the object to be measured 117 is reflected by the lens 118.
It leads to a tailed interferometer 119.

In this embodiment, a Wollaston prism is shown as the sensitometer. The vicinity of the center of this Wollaston prism and the slit opening 114 are in a conjugate relationship,
A slit aperture image is formed near the center of the Wollaston prism. Since white interference fringes are formed in the slit Sekiguchi image light east near the center of the Wollaston prism,
The interference fringes are imaged onto a one-dimensional photodiode array 122 by an imaging lens 121 via a polarizing plate 120. Since the image of the slit Sekiguchi 114 is also formed on this photodiode array 122, by making the size of this slit Sekiguchi image smaller than the size of the light receiving surface of the photodiode array 122, the light amount of the white interference fringes can be reduced. can be guided onto the photodiode array 122 without loss. In the case of this embodiment, since the white interference fringes are guided onto the photodiode array 122 by the normal imaging lens 121 system, aberrations such as lens distortion are more likely to be avoided than using the anamorphic lens system of the previous embodiment. The impact can be reduced. Note that the polarization direction of the polarizing plate in this embodiment forms an angle of 45° with the optical axis of the Wollaston prism, as in the previous embodiment.

In the description of the embodiments of the present invention, a white light source is shown as a light source, but the light source may be a visible light source or an invisible light source as long as it has a wavelength width.

As described above, according to the present invention, thickness and refractive index can be measured in a non-destructive and non-contact manner compared to conventional thickness measurement methods.
Further, in addition to the fact that the measurement time is extremely short, it has the advantage that it is possible to construct an apparatus that has no moving parts at all, and that it is possible to construct a compact apparatus.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams explaining the principle of the present invention, Figure 3,
FIG. 4 is a diagram for explaining a white interference fringe waveform, FIGS. 5 and 6 are diagrams for explaining the first and second embodiments of the present invention, and FIG. 7 is a diagram for explaining the third embodiment. FIG. 8 is a diagram for explaining the fourth embodiment, and FIGS. 9 and 10 are diagrams for explaining the fifth embodiment. In the figure, 1 is a white light source, 2 is a white light source, 3 is an object to be measured, and 5 is a white light source.
is the light reflected on the first surface, 6 is the light reflected on the second surface,
7 is a light splitter, 9 is a mirror, 11 is a troubled mirror, 1
2 is Shins, 13 is Screen. Figure 2 Figure 3 Inferior design Figure 5 Figure 5 Tsutomama Figure Q Figure Kaya 8 Figure ! 0 figure

Claims (1)

[Claims] 1. A plurality of wavefronts of light having a phase shift that includes information such as the thickness and refractive index of the measured object from the measured object illuminated by a light beam having a wavelength width are divided, and each division is performed. In a method for measuring physical constants such as the thickness and refractive index of a measured object by overlapping the divided wavefronts, the divided wavefronts are relatively tilted and overlaid to form interference fringes, and the interference fringes are A method for measuring physical constants of an object to be measured, characterized in that the measurement is carried out from a position of .