JP2000146836A - Refractive index measuring method utilizing transmission phenomenon of evanescent wave and its measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a refractive index measuring method and its measuring device utilizing a transmission phenomenon of an evanescent wave, capable of measuring the refractive index of a substance in a thin film state without using thickness data of a sample and without needing complicated arithmetic processing that uses data actually measured. SOLUTION: A substance B of a refractive index lower than that of a substance A is brought into close contact with a flat surface of the substance A of a known refractive index, the substance B is adapted to be a thin film of a known refractive index and a known film thickness smaller than an ooze-out distance D of an evanescent wave 3 generated when measuring light 1 incoming from the side of the substance A is totally reflected by an interface 2 bordering the substance B, a sample (substance C) whose refractive index is to be found is brought into close contact with the substance B, an evanescent wave 3 is generated by totally reflecting measuring light 1 by the interface 2 between the substance A and the substance B, and the refractive index of the substance C is found by finding the size of a part of the evanescent wave 3, passing through the substance B, oozed out into the substance C, and changed to transmitted light 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring a refractive index utilizing a transmission phenomenon of an evanescent wave, which is particularly effective for measuring the refractive index of a material used in the field of optical communication devices and optical sensing. It is.

[0002]

2. Description of the Related Art Usually, when measuring the refractive index of a material, an ellipsometer or a refractometer is used. Abbe's refractometer can measure the refractive index of a sample by pressing a prism with a higher refractive index than the sample onto the sample and irradiating light from the prism side and measuring the critical angle. It cannot be applied to materials other than materials. Further, as another method of measuring the refractive index, in the case of a sample having a known film thickness, it can be obtained by making light of a specific wavelength incident on the substance to be measured and calculating the transmitted light and reflected light. In addition, as a method of measuring using an evanescent wave which is a seepage in total reflection, there are refractometers such as a surface plasmon sensor, an optical waveguide sensor, and a light leakage mode sensor, and in these methods, the thickness of a sample is known. No need to be.

[0003]

However, in such a conventional method of measuring the refractive index and its measuring apparatus, in the case of a measuring method which requires accurate thickness data of the substance to be measured, it is necessary to adjust the wavelength to the wavelength of the measuring light. A sample having a different thickness is required. Therefore, as a step before measuring the refractive index,
Work to measure the film thickness and polish the sample for thickness adjustment was required. Also, when measuring a material having a shape that is very difficult to measure the film thickness, it is necessary to reshape the shape, and the refraction index of the reshaped material is different from the value of the material that is actually required. I was afraid.

In the method of using an evanescent wave for measuring the refractive index, it is not necessary to measure the film thickness. However, since the measurement is performed by directly pressing the sample to be measured on the optical waveguide portion, complicated calculation processing and device configuration are always required. is necessary. further,
In the case of a surface plasmon sensor, it is necessary to install an angle variable mechanism in the part where the measuring light is introduced and in the detector.In the case of an optical waveguide sensor, a special optical system must be used because the phase of the light must be obtained. Because of the necessity, a large-scale special device was required for the measuring device. Further, in the light leakage mode sensor, since the measurement light is directly introduced into the sample, there is a problem that the measurement becomes impossible if there is no absorption by the sample.

In recent years, in the field of communication and environmental sensing, attention has been focused on the use of light in a long wavelength infrared light region. Along with this, there is a desire to know the refractive index of light in the infrared region of the material, the refractive index of the infrared light transmitting material itself, and the refractive index of the material as a thin film rather than a bulk material. . However, it is difficult to construct a measurement system for measuring the refractive index of light in the infrared light region, and particularly for measuring the refractive index of a long-wavelength infrared light transmitting material, based on a conventional measurement method. Was. This is because, when long wavelength infrared light is used as the measurement light, the refractive index of the optical material for introducing the measurement light is high, and the measurement light is directly introduced into the sample. Furthermore, since some materials that transmit infrared light having a long wavelength of several μm or more have toxicity to the human body, there is also a desire to avoid processing work for adjusting the thickness, When used as a monitor or when incorporated as a refractive index sensor into other devices, there has been a demand to reduce the size, but there is a problem that they cannot be answered.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it has been proposed that a thin film sample can be obtained without using thickness data of the sample and without complicated calculation processing using actually measured data. An object of the present invention is to provide a refractive index measuring method using an evanescent wave transmission phenomenon capable of measuring a refractive index, and a measuring device therefor.

[0007]

In order to achieve the above object, according to the present invention, a first material having a known refractive index through which a measurement light is transmitted is provided on a flat surface of the first material, and the first light is transmitted through the first material. The second material having a lower refractive index than that of the first material.
A thin film having a known refractive index and a known film thickness smaller than the exudation distance of an evanescent wave generated when the measurement light incident from the material side is totally reflected at the interface with the second material is brought into close contact. A sample whose refractive index is to be obtained is brought into close contact with the thin film, and an evanescent wave is generated by totally reflecting measurement light at an interface between the first substance and the thin film. Then, the magnitude of the evanescent wave that oozes into the sample and changes into transmitted light is determined to determine the refractive index of the sample.

[0008] Further, the intensity of the measurement light which is totally reflected back at the interface between the first substance and the thin film when the sample is not present, and the intensity of the first substance and the first substance when the sample is set. The magnitude of the evanescent wave that oozes into the sample and changes into transmitted light is determined from the difference from the intensity of the measurement light that returns after being totally reflected at the interface with the thin film.

Further, the measuring light is guided through the first material so that total reflection of the measuring light at the interface between the first material and the thin film is repeated many times.
And the intensity is measured.

A prism for guiding an optical path of measurement light is provided in a sample chamber of a Fourier transform infrared photometer having an infrared light source, a Michelson interferometer, a detector, and a sample chamber, and the prism is brought into close contact with the prism. A first substance having a known refractive index and a second substance having a lower refractive index than the first substance adhered on the first substance and having a film thickness smaller than a seepage distance of an evanescent wave. A refractive index measuring device having a thin film is used.

Further, silicon is used for the first substance,
SiO ₂ is used as the second substance.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a conceptual diagram showing an embodiment of a refractive index measuring method using an evanescent wave transmission phenomenon according to the present invention. As shown in FIG. 1A, a first substance (substance A) having a known refractive index that transmits the measuring light 1 and a substance A
When the measurement light 1 is totally reflected at the interface 2 with the second substance (substance B) having a lower refractive index, an evanescent wave 3 is generated and seeps into the substance B. The evanescent wave 3 is exponentially attenuated while the interface 2
From the substance B.

[0013]

(Equation 1)

Here, E is the intensity of the evanescent wave 3 when the distance from the interface 2 is d, E ₀ is the intensity when d = 0,
n ₁ and n ₂ are the refractive indices of substance A and substance B, and n _ij is n _i
/ N _j indicates a relative refractive index. D is the distance (leaching distance) when the intensity of the evanescent wave 3 has attenuated to 1 / e (the base of the natural logarithm), and is defined by Expression 2. This is a value that depends on the wavelength λ of the incident light.

[0015]

(Equation 2)

Further, as shown in FIG. 1B, the substance B is formed into a thin film having a thickness smaller than the seeping distance D of the evanescent wave 3, and another substance C ( When the refractive index n ₃ ) exists, the evanescent wave 3 reaches the further outer material C, and becomes a sample (material) from which the material C is measured. This is the transmission phenomenon of the evanescent wave, and the intensity ratio T of the light transmitted to the substance C with respect to the incident light is expressed by Expression 3.

[0017]

(Equation 3)

Here, Ts and Tp represent the s-polarized component (the component perpendicular to the incident surface) and the p-polarized component (the component included in the incident surface) of the incident light, respectively. It is obtained by the following equation (4).

[0019]

(Equation 4)

Here, p _s , p _o , q _s , q _o are as shown in Formula 5, and m _p 11, m _p 12, m _p 21,
_{m p 22, m s 11,} m s 12, m s 21, m s 22 is a factor of characteristic matrix Mp and Ms of equation (6).

[0021]

(Equation 5)

[0022]

(Equation 6)

Here, γ _d , p and q are expressed by the following equation (7).
z is the thickness of the substance B, and i is the imaginary root.

[0024]

(Equation 7)

Here, when n ₁ , n ₂ , z and the incident angle θ ₁ are known, Tp and Ts in the equation 3 are the refractive index (n ₃ ) of the substance C.
Is a function of Therefore, to determine n ₃ ,
You don't need to know the thickness. The substance B is included in the wavelength λ of the measurement light.
It is necessary to apply one that is in a range where strong absorption due to is not caused. The wavelength range of light that can be used is limited by the materials used as the substances A and B, and the range of n _{3 that} can be measured is limited by n ₁ , n ₂ , z, and θ ₁ . Therefore, in designing a measurement system, it is important to select an optical material to be used and a combination thereof. Further, the value of z, which is the thickness of the substance B, needs to be sufficiently smaller than the value of D, which is a reference of the penetration depth of the evanescent wave. Regarding the relationship between the transmitted light intensity ratio T and the refractive index n ₃ , λ =
2.0 (μm), n ₁ = 3.45 (silicon), n ₂ =
1.46 (SiO ₂ ), z = 100 (Å), incident angle θ ₁ =
FIG. 2 shows a result obtained by substituting 45 °. As shown in the figure, since a correlation is recognized between T and n ₃ , if T is measured, the refractive index n _{3 of the} substance C can be obtained.

Next, the measuring method will be described. First, a substance A and a substance B having a known refractive index are prepared. At this time, it is assumed that the substance B has a sufficiently small thickness and is in close contact with the substance A. At this time, it is set so that the measurement light 1 is introduced at an angle θ ₁ at which the measurement light 1 is totally reflected at the interface 2 between the substance A and the substance B. Here, the values of n ₁ , n ₂ , z, and θ ₁ are substituted into Expressions 3 to 7, and
A curve representing the relationship between T and n ₃ as shown in FIG. 2 is obtained.
The value of Q, which is the actual measured value of T, is the amount of light that has changed into the transmitted light 4 by the substance C, and is determined from the output value P when the detector 5 detects the light. A standard sample with a known refractive index is measured as the substance C, and the output value P of the detector 5 at that time is obtained. The maximum value Pmax of the output value P of the detector (the detector 5 when there is no substance C)
Output value) minus output value P (Pmax-P)
Is an actual measurement value Q. The measured value Qi of the standard sample is applied to the curve obtained as shown in FIG. 2, and a calibration curve representing the correlation between the T value of the curve and the measured value Qi is created. After the calibration curve has been created, a sample, which is an unknown sample, is set as the substance C, and the output value P of the detector 5 at that time is subtracted from Pmax to obtain an actually measured value Qs. The actually measured value Qs is applied to the previously obtained calibration curve, and the refractive index is obtained from the horizontal axis. However, when the substance C has a refractive index that satisfies the condition of total reflection when brought into contact with the substance A, the measured value shows a value close to zero. In the apparatus for measuring parameter values as used in FIG. 2, when the refractive index is 2.4 or less, the substance A
And total reflection occurs. Therefore, when the measured value is close to zero in FIG. 2, the refractive index cannot be obtained, but n
It can be seen that ₃ is a value smaller than 2.4.

An example in which the refractive index of a material that transmits infrared light of a long wavelength, which has recently attracted attention in the field of infrared light communication and sensing using near-infrared light and mid-infrared light, is described.

FIG. 3 is a block diagram showing an embodiment of a refractive index measuring apparatus utilizing the transmission phenomenon of an evanescent wave according to the present invention. As shown in the figure, selenization for guiding the optical path of the measurement light 1 into a sample chamber 9 of a Fourier transform infrared photometer having an infrared light source 6, a Michelson interferometer 7, a detector 8, and a sample chamber 9. A prism 10 made of zinc, a first substance (substance A) having a known refractive index adhered on the prism 10, and a substance A
A second sample (substance B) having a lower refractive index than the substance A adhered thereon and having a film thickness smaller than the exudation distance D of the evanescent wave, and a measurement sample (substance C) fixed on the substance B I have.

Since the infrared light transmitting material has a large refractive index, a measuring system for measuring a substance having a refractive index of 2.0 or more is constructed.
The Fourier transform infrared photometer emits infrared light from an infrared light source 6, passes through a slit and is introduced into a Michelson interferometer 7, from which a sample (material C) placed in a sample chamber 9 is provided. The measuring light 1 introduced and passed through the sample chamber 9 is detected by the detector 8 and its intensity can be measured. That is, when the measuring light 1 is converted into the transmitted light 4 in the sample, the intensity of the measuring light 1 detected by the detector 8 decreases.

Therefore, a measuring apparatus as shown in FIG. 3 was constructed. A zinc selenide prism 10 having an incident angle of 45 degrees was set in a sample chamber 9 of a Fourier transform infrared photometer so that the measurement light 1 could be introduced perpendicularly to the cut surface thereof.
A silicon wafer on which O ₂ is formed is fixed on a prism 10 of zinc selenide so that the upper surface becomes SiO ₂ . That is, the silicon wafer becomes the substance A, and the SiO ₂ becomes the substance B. Next, a substance C whose refractive index is to be obtained is pressed onto SiO ₂ . Therefore, when the transmitted light 4 occurs in the material C portion, the intensity detected by the detector 8 decreases. Since the wavelength of the infrared light that can pass through the silicon wafer is 1.0 to 6.2 μm, the value of D (the oozing distance of the evanescent wave) is 0.18 to 1.15 μm in the measurement system of FIG. Become. Material B SiO
₂ is manufactured while controlling the film thickness by a thermal oxidation method, and the thickness is set to 150 ° (refractive index 1.4) sufficiently smaller than the value of D.
6). At this time, the incident angle θ ₁ of the measurement light 1 from the substance A to the substance B is about 29 degrees, which is larger than the critical angle defined by the refractive indexes of the silicon wafer and SiO ₂ . Therefore, the condition of total reflection at the interface 2 between the silicon wafer and SiO ₂ is satisfied. The measurable refractive index range also depends on the upper interface 2 of the substance A (silicon) in this case.
Is a substance having a refractive index of 1.7 or less, and therefore is 1.7 or more. The infrared light applicable as the measurement light must be in a wavelength range that can transmit silicon, and therefore has a long wavelength up to 6.2 μm.

Next, a specific measurement procedure will be described. First, the absorption energy value of a standard sample for preparing a calibration curve is measured. A measurement system in which a standard sample is set as a substance C on SiO ₂ is set in a sample chamber 9 of a Fourier transform infrared spectrometer. The standard samples include mirror-polished silicon crystal (refractive index ≒ 3.4), germanium crystal (refractive index ≒ 4.0), and zinc selenide crystal (refractive index ≒ 2.4).
Was applied. When an absorption measurement is performed using a Fourier transform infrared spectrometer, an energy output value can be obtained over a wide wavelength range of infrared light as shown in FIG. Therefore,
The wavelength of the light whose refractive index is to be obtained is arbitrarily selected, the output value P at a certain wavelength is read, and this value is subtracted from the maximum value Pmax = 100 of the detector output value to obtain the actually measured value Qi of the transmitted light intensity ratio. . The measured values Qi of the three standard samples thus obtained are fitted to a curve obtained from the equation (7) to determine a calibration curve.

FIG. 5 shows a calibration curve created by the above procedure. Here, light having a wavelength of 3.70 μm is used,
The values substituted into Equation 7 are λ = 3.70 (μm) and n ₁ =
3.42, n ₂ = 1.46, z = 150 (Å).
The Qi of the standard sample is shown in the plot.

Next, the refractive index of an unknown sample silicon carbide thin film (substrate is a silicon wafer) produced by the plasma CVD method was determined. Silicon carbide thin film S
It is arranged so as to be in contact with iO _2, and the output value Q of this sample is
s was determined. Qs of the silicon carbide film at a wavelength of 3.7 μm was 57.96. When this value was applied to the calibration curve in FIG. 5, the refractive index was determined to be 2.58. The refractive index of bulk silicon carbide is approximately 2.53 according to the literature. Here, the reason that the refractive index of the silicon carbide thin film of the sample was obtained as a value higher than the literature value is that the silicon element is present in the silicon carbide film of the sample at a ratio larger than the stoichiometric ratio. Therefore, the obtained value is an appropriate value.

As described above, it has been shown that the refractive index of an unknown sample can be measured without measuring the film thickness and the film can be applied as a material analysis means.

As is evident from the above measurement results, the deviation of the plot of the standard sample to be measured for the preparation of the calibration curve is a measurement error as it is. Although the error is large compared to the Abbe refractometer, the measurement of the film thickness is difficult. This is a highly convenient method in that the refractive index can be easily measured without performing it.

As described above, the measurement of the thickness of the sample, the work of adjusting the thickness, and the complicated calculation processing are unnecessary, and the refractive index can be maintained even if the sample itself has no absorption without configuring a special large apparatus. A measurable refractive index measurement method was devised. Unlike conventional methods that use total reflection of light by the sample itself or light that passes through the sample, evanescent light generated from the total reflection light generated at the interface between the optical waveguide portion and the substance other than the sample is different from the conventional method. This is a method in which the absorption intensity of a wave by a sample is determined, and the refractive index is determined using the absorption intensity.
In addition, using this method, a material that can transmit long-wavelength infrared light with a high refractive index can be used to measure the refractive index of light in the infrared region based on a commercially available infrared photometer. A rate measuring device was newly devised.

That is, in the refractive index measuring method utilizing the transmission phenomenon of the evanescent wave, the refractive index n _{3 of the} sample is obtained from the transmitted light intensity ratio T of the sample (substance C). without, also without requiring a complicated calculation processing using the actually measured data, it becomes possible to measure the refractive index n ₃ of the sample of the thin film state. Further, it is necessary to construct a measurement system in consideration of the measurement conditions of the sample, but the total reflection condition of the measurement light does not depend on the refractive index n _{3 of the} sample. Therefore, once the measurement system is constructed and the calculation is performed, there is an advantage that it can be easily applied to the measurement of various kinds of substances. In addition, since it is not necessary to prepare a special sample for film thickness measurement and the sample can be measured in the same shape, it can be applied as a process monitoring measurement method. Useful for correlated materials.

Further, in the above-described refractive index measuring device utilizing the transmission phenomenon of the evanescent wave, a commercially available infrared spectrometer can be combined to easily perform the refractive index measurement using light in the infrared region. Even a highly toxic infrared light transmitting material can measure the refractive index without special processing. Also, measurement can be performed using an optical meter whose wavelength of the measurement light is not variable. Therefore, a refractive index measuring device constructed by applying such a refractive index measuring method is suitable for being incorporated into a small system in the future. Further, it is possible to apply a waveguide portion formed by cutting out a silicon wafer as a disposable part. In the future, it can be used as one of the development support devices for communication systems and sensing systems using infrared light. Further, by simply incorporating the configuration of the sample chamber portion into a commercially available ultraviolet / visible spectrometer, it is possible to configure an apparatus that uses not only the infrared region but also light having a wavelength of several hundred nm as the measurement light.

[0039]

As described above, in the refractive index measuring method utilizing the transmission phenomenon of the evanescent wave according to the present invention and the measuring apparatus, the magnitude of the evanescent wave oozing out into the sample and changing into the transmitted light is obtained. The refractive index of the sample can be measured without using the thickness data of the sample and without complicated calculation processing using the actually measured data. It is.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an embodiment of a refractive index measuring method using an evanescent wave transmission phenomenon according to the present invention.

FIG. 2 is a relationship diagram between a transmitted light intensity ratio and a refractive index.

FIG. 3 is a configuration diagram showing an embodiment of a refractive index measuring device utilizing an evanescent wave transmission phenomenon according to the present invention.

FIG. 4 is a diagram showing an absorption measurement result of a silicon standard sample.

FIG. 5 is a calibration diagram illustrating a relationship between a transmitted light intensity ratio and a refractive index.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Measurement light 2 Interface 3 Evanescent wave 4 Transmitted light 5 Detector 6 Infrared light source 7 Michelson interferometer 8 Detector 9 Sample room 10 Prism

Claims (5)

[Claims] A first material having a known refractive index that transmits the measuring light, and a second material having a lower refractive index than the first material that transmits the measuring light and having a lower refractive index than the first material; A thin film having a known thickness and a refractive index smaller than the exudation distance of the evanescent wave generated when the measurement light incident from the first material side is totally reflected at the interface with the second material is adhered to the thin film. Then, a sample whose refractive index is to be obtained is brought into close contact with the thin film, and the measurement light is totally reflected at the interface between the first substance and the thin film to generate an evanescent wave. A refractive index measuring method using a transmission phenomenon of an evanescent wave, wherein the magnitude of an evanescent wave that has transpired into the sample and changed into transmitted light is determined to determine the refractive index of the sample. 2. The method according to claim 1, wherein the intensity of the measuring light which is totally reflected back at the interface between the first material and the thin film when the sample is absent and the intensity of the first material and the first material when the sample is set. 2. The magnitude of an evanescent wave that oozes into the sample and is converted into transmitted light from a difference from the intensity of the measurement light that is totally reflected at the interface with the thin film and returns. A refractive index measurement method using the transmission phenomenon of evanescent waves. 3. The measurement light is guided through the first material so that total reflection of the measurement light at the interface between the first material and the thin film is repeated many times. The refractive index measurement method using the transmission phenomenon of an evanescent wave according to claim 2, wherein the intensity is measured by emitting light from the substance. 4. An infrared light source, a Michelson interferometer, a detector,
A prism for guiding an optical path of measurement light, a first substance having a known refractive index closely adhered on the prism, and a first material having a refractive index in the sample chamber of a Fourier transform infrared photometer having a sample chamber;
A second material having a lower refractive index than the first material adhered onto the material and having a smaller film thickness than the oozing distance of the evanescent wave.
A refraction index measuring device utilizing an evanescent wave transmission phenomenon, characterized in that the device has a thin film made of the following materials. 5. The method according to claim 4, wherein silicon is used as said first material and SiO ₂ is used as said _second material.
2. A refractive index measuring device utilizing the transmission phenomenon of an evanescent wave described in 1.