JP4036334B2 - Electro-optic constant measuring method and apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electro-optic constant measuring method and apparatus for measuring the electro-optic effect of a thin film at multiple wavelengths.
[0002]
[Prior art]
The electro-optic effect is a phenomenon in which the refractive index and the like change depending on the applied voltage, and optical communication devices using this electro-optic effect are currently in wide use. In many cases, a single crystal is used, and in particular the Pockels constant of a typical optical crystal at the wavelength of a HeNe laser is known. Usually, since a bulk optical crystal has a size of several mm square or more, several methods for measuring an electro-optic effect using laser light have been proposed and actually used.
[0003]
The most common measurement methods are as follows. The laser light is divided into two by a half mirror, one is passed through an electro-optic crystal to which voltage is applied, and the other is used as a reference signal to optically interfere with each other, and the intensity change is examined. It is possible to calculate the electro-optic constant from the applied voltage when the phase is shifted by a half wavelength [Non-Patent Document 1]. In this measurement method, it is important that the size of the electro-optic crystal is larger than the spot diameter of the laser.
[0004]
On the other hand, since the electro-optic constant of the thin film depends on the film quality, the value of the bulk single crystal cannot be applied, and it is necessary to measure each thin film. However, in order to apply the above measuring method to a thin film, it is necessary to produce an optical waveguide. If a Mach-Zehnder type optical waveguide is formed on a thin film, a voltage is applied to one of the optical waveguides after bifurcation, and the change in signal intensity due to light interference is used, the same measurement is performed. In principle it is possible.
[0005]
However, it is necessary to make a diffusion type optical waveguide or to make a ridge type optical waveguide by etching to make sure that light is guided without interference to some extent, and that light enters only the thin film optical waveguide. However, in order to pick up only the light that has passed through the thin-film optical waveguide, there are many problems that must be cleared, such as the need for spot size conversion.
[0006]
In the case of a material having birefringence, the laser light is polarized through a 45-degree polarizing plate so that the TM polarization and the TE polarization are equal, and passed through the electro-optic thin film in a state where a voltage is applied. After that, if the polarization information is taken out by a 45-degree polarizing plate, information on the electro-optic constant can be obtained [Non-Patent Document 2]. However, even in this case, it is necessary to make the thin film into a waveguide, to guide light without being attenuated, and the like.
[0007]
Further, in the measurement using these single wavelength lasers, there is a limitation that only an electro-optic constant corresponding to the measurement wavelength can be obtained.
[0008]
Non-patent document 1
JL Stevenson, J. Phys. D: Appl. Phys. 6, L13 (1973).
Non-Patent Document 2
GE Peterson, AA Ballman, PV Lenzo, and PM Bridenbaugh, Appl. Phys. Lett. 5, 62 (1964).
[0009]
[Problems to be solved by the invention]
By the way, if the electro-optic constant at each wavelength can be measured in the state of a thin film without producing an optical waveguide, it can be determined in advance whether the thin film has the physical property values required as an electro-optic thin film. The obtained electro-optic constant can be used for element design. Further, if the electro-optic constant can be measured in a continuous wavelength range using a continuous light source such as a halogen lamp instead of a single wavelength measurement by laser light, other information on selection of the design wavelength can be obtained.
[0010]
However, the above-mentioned waveguide type measurement requires the use of laser beams having high intensity and uniform phase, and it has been said that it is not easy to measure electro-optic constants by other methods.
[0011]
The present invention solves the above-described problems and is effective in evaluating the magnitude of the electro-optic effect of the thin film necessary for designing an optical waveguide and a functional optical component incorporating the electro-optic thin film. An object is to provide a constant measuring method and apparatus.
[0012]
[Means for Solving the Problems]
In order to solve the above problems, the invention of claim 1 is capable of applying a voltage between the substrate and the transparent conductive thin film by covering the electro-optic thin film formed on the substrate with the transparent conductive thin film. In the state, when the spectrum of the detected light wavelength of (tan Ψ, cos Δ) is measured by spectroscopic ellipsometry, and the tan Ψ spectrum at the time of voltage application and non-application is obtained by measurement, the amplitude of the difference spectrum between the two tan Ψ spectra By comparing the amplitude of the reference spectrum obtained by differentiating the tan Ψ spectrum with respect to the detection light wavelength, the effective wavelength change of the optical interference caused by the voltage application is calculated, and the refractive index change is calculated from the calculated value. An electro-optic constant measuring method characterized in that the electro-optic constant is calculated by deriving a minute.
According to a second aspect of the present invention, an electro-optic thin film formed on a substrate is covered with a transparent conductive thin film, and a voltage can be applied between the substrate and the transparent conductive thin film by a spectroscopic ellipsometry method. From the measuring device that measures the detection light wavelength-dependent spectrum of (tan Ψ, cos Δ) , the amplitude of the difference spectrum between the two tan Ψ spectra from the tan Ψ spectrum when the voltage is applied and not applied, and the By comparing the amplitude of the reference spectrum obtained by differentiating the tan Ψ spectrum with respect to the detection light wavelength, the effective wavelength change of optical interference caused by voltage application is calculated, and the refractive index change is derived from that value. Then, the electro-optic constant measuring device is provided with an arithmetic device for calculating the electro-optic constant.
[0013]
In the present invention, the electro-optic constant of the thin film is measured using a spectroscopic ellipsometry method. From the ratio ρ of Fresnel reflectivity (R _p and R _s ) of p-polarized light and s-polarized light, the ellipso angles Ψ and Δ are
ρ = R _p / R _s = tan Ψ exp (iΔ)
It can be obtained from the following formula. In actual measurement, the values of tan Ψ and cos Δ are measured in a certain wavelength range to obtain a spectrum. Here, tan Ψ corresponds to the amplitude, cos Δ corresponds to the phase portion, and information on the refractive index change is mainly included in tan Ψ.
[0014]
However, the refractive index change is as small as 10 ⁻³ to 10 ⁻⁵ at most even under an electric field close to the coercive electric field of the electro-optic thin film. For this reason, when the tan Ψ spectra when voltage is applied and when no voltage is applied are displayed on the same graph, they almost completely overlap.
[0015]
However, if the difference spectrum between the two is taken, a significant intensity difference can be obtained spectrally. By comparing the amplitude of the difference spectrum with the amplitude of the reference spectrum obtained by differentiating the tan Ψ spectrum, it is possible to estimate the effective wavelength change of optical interference caused by voltage application. Then, a refractive index change Δn corresponding to the effective wavelength change is derived,
Δn = nr _eff ³ E / 2
The effective electro-optic constant (r _eff ) of the thin film can be obtained according to the following equation. In this equation, E is the electric field strength.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described in detail with reference to the drawings. An electro-optic constant measuring device according to this embodiment is shown in FIG. The electro-optic constant measuring apparatus in FIG. 1 includes an optical system 10, a spectroscope 20, and an arithmetic device 30. This electro-optic constant measuring apparatus measures the electro-optic constant of the sample 100.
[0017]
In the present embodiment, the ellipso angle Ψ of the sample 100 is measured using a spectroscopic ellipsometry method. This measuring apparatus is the optical system 10 and the spectrometer 20. The optical system 10 includes an adjustment table 11, a light source unit 12, and a light receiving unit 13. The adjusting table 11 sets the angle θ between the light source unit 12 and the light receiving unit 13 so that the light from the light source unit 12 is incident on the sample 100 on the sample table 11A at a predetermined angle. In the present embodiment, the angle θ is 75 degrees.
[0018]
The light source unit 12 makes polarized light incident on the sample 100. For this reason, in the light source unit 12, the ultraviolet to visible light emitted from the xenon lamp 12A is reflected by the mirrors 12B and 12C, and the polarizer 12D is polarized by 45 degrees of polarization.
[0019]
The light receiving unit 13 receives light reflected from the sample 100. For this purpose, in the light receiving unit 13, the analyzer 13A detects the polarization state of the reflected light. The mirror 13B sends the detection light from the analyzer 13A to the transmission unit 13C. The transmission unit 13C guides the detection light to the spectrometer 20.
[0020]
The spectroscope 20 performs spectrum analysis of the detection light from the light receiving unit 13. That is, the detection light is measured while changing the detection wavelength. The spectroscope 20 converts the analyzed light into an electrical signal by the photomultiplier tube 21 and sends it to the arithmetic unit 30.
[0021]
The arithmetic unit 30 calculates an electro-optic constant from the analysis light from the spectrometer 20. That is, the arithmetic unit 30 measures the electro-optic constant using a spectroscopic ellipsometry method.
[0022]
From the ratio ρ of Fresnel reflectivity (R _p and R _s ) of p-polarized light and s-polarized light, the ellipso angle Ψ and the phase difference Δ are
ρ = R _p / R _s = tan Ψ exp (iΔ)
It is requested from. In actual measurement, the values of tan Ψ and cos Δ are measured in a certain wavelength range to obtain a spectrum. tan Ψ corresponds to the amplitude, and cos Δ corresponds to the phase portion. Information on the refractive index change is mainly included in tan Ψ.
[0023]
The change in refractive index is very small, about 10 ⁻³ to 10 ⁻⁵ , even under an electric field close to the coercive field of the electro-optic thin film. Therefore, the tan Ψ spectrum when voltage is applied and when no voltage is applied is displayed on the same graph. In that case, they will almost completely overlap.
[0024]
For this purpose, the arithmetic unit 30 calculates the difference between the tan Ψ spectra when the voltage is applied and when the voltage is not applied. By taking the difference spectrum between the two, a significant intensity difference can be obtained spectrally. Furthermore, the arithmetic unit 30 differentiates the tan Ψ spectrum to obtain a reference spectrum.
[0025]
Thereafter, the computing device 30 computes the effective wavelength change of optical interference caused by voltage application by comparing the amplitude of the difference spectrum of the tan Ψ spectrum with the amplitude of the reference spectrum of the tan Ψ spectrum. Then, the arithmetic unit 30 derives a refractive index change Δn corresponding to the effective wavelength change,
Δn = nr _eff ³ E / 2
The effective electro-optic constant (r _eff ) of the thin film is obtained according to the following equation. In this equation, E is the electric field strength.
[0026]
The above is the configuration of the electro-optic constant measuring apparatus according to the present embodiment. Next, an electro-optic constant measuring method using the electro-optic constant measuring apparatus will be described. As a sample used here, as shown in FIG. 2, a LiNbO ₃ oriented film 102 which is a 300 nm-thick C-axis oriented LiNbO ₃ crystal thin film is formed on a low-resistance Si substrate 101, and a leak is formed thereon. In order to reduce the current and increase the applied voltage, a 50 nm thick SiO ₂ protective film 103 was formed. Thereafter, a TiN transparent conductive film 104 having a thickness of 20 nm was formed as an upper electrode, and finally, an Al film was deposited on the back surface of the low resistance Si substrate 101 to form an Al lower electrode 105.
[0027]
A variable voltage 200 is applied to the sample 100 between the TiN transparent conductive film 104 and the Al lower electrode 105.
[0028]
The sample 100 is set on the sample table 11A of the adjustment table 11 of the electro-optic constant measuring apparatus. The ultraviolet to visible light emitted from the xenon lamp 12A of the optical system 10 is polarized into 45-degree polarized light by the polarizer 12D and is incident on the measurement sample 100 set on the sample stage 11A. The polarization state of the light reflected from the sample 100 is detected by the analyzer 13A of the light receiving unit 13. When measurement is performed while the detection wavelength is changed by the spectroscope 20, a (tan Ψ, cos Δ) spectrum is obtained.
[0029]
FIG. 3 is a (tan Ψ, cos Δ) spectrum of the sample 100. In FIG. 3, the solid line is tan Ψ and the dotted line is cosΔ. The incident angle θ of the probe light on the sample 100 was fixed at 75 °.
[0030]
The vibration structure corresponding to the wavelength is caused by optical interference at the interface of the multilayer film. When the voltage is applied to + 5V, + 10V, and + 15V, the refractive index change is too small, and the vertical axis scale in FIG. 3 completely overlaps the spectrum of 0V and cannot be distinguished.
[0031]
Here, assuming that the usual spectroscopic ellipsometry analysis method is applied, the spectrum is simulated assuming a certain structural model (tanΨ, cosΔ), and the parameters are optimized to match the spectrum obtained in the experiment. Turn into. As a result, the film thickness, refractive index, and absorption coefficient of each layer are obtained, and the change in refractive index of the electro-optic crystal thin film before and after voltage application is obtained.
[0032]
However, in this spectroscopic ellipsometry analysis method, the error of each parameter determined by optimization is 0.1% or more, and the refractive index values slightly different before and after voltage application are subtracted from each other. It is practically difficult to obtain a reliable value of refractive index change due to the loss of digits.
[0033]
FIG. 4 shows the difference spectrum [tanΨ (xV) −tanΨ (0V)] obtained by subtracting the tanΨ spectrum at a voltage of 0 V from the tanΨ spectrum at the time of applying each voltage.
It is. The noise level of the signal is within ± 0.001, and the significant vibration structure in FIG. 4 is clearly caused by the electro-optic effect. Further, the fact that the amplitude increases almost in proportion to the applied voltage means a primary electro-optic effect (Pockels effect).
[0034]
On the other hand, the wavelength of the tan Ψ spectrum with a voltage of 0 V is shifted by a step size of 0.005 μm,
[Tan Ψ (λ) −tan Ψ (λ + 0.005 μm)]
FIG. 5 shows a differential reference spectrum obtained by calculating. The reference spectrum of FIG. 5 has a vibration shape similar to the difference spectrum of FIG. 4, but only the amplitude is different. This is because a minute refractive index change Δn caused by voltage application effectively corresponds to a minute change in wavelength.
[0035]
Here, as an example, the electro-optic constant at a wavelength of 0.4 μm is evaluated. From the reference spectrum of FIG. 5, the peak-to-peak amplitude (maximum value−minimum value) near the wavelength of 0.4 μm is 0.19. This is an amplitude value corresponding to a wavelength change of 0.005 μm. From the difference spectrum in FIG. 4, the peak-to-peak amplitude of the difference spectrum at a voltage of 15 V is 0.015. From these two values 0.19 and 0.015, the effective wavelength change of the measurement sample is
Δλ = 0.005 × (0.015 ÷ 0.19) = 3.95 × 10 ⁻⁴
It is.
[0036]
Since light in a medium with a refractive index n behaves as a wavelength λ / n, Δλ and Δn are
Δn / n = Δλ / λ
Are tied together. From this relationship and the previously calculated Δλ value of 3.95 × 10 ⁻⁴ ,
_{Δn = (Δλ / λ) n} e = 2.3 × 10 -3
Is required. Since the incident angle is 75 °, the measured refractive index is almost close to n _e = 2.286. Since Δn = 2.3 × 10 ⁻³ , the Pockels constant r _eff is
r _eff = 2Δn / n _e ³ E = 7.7 pm / V
Is determined.
[0037]
Thus, according to the present embodiment, if the difference spectrum (FIG. 4) of the tan Ψ spectrum when the voltage is applied and when the voltage is not applied is taken, a significant intensity difference can be obtained spectrally. By comparing the amplitude of the difference spectrum with the amplitude of the reference spectrum (FIG. 5) obtained by differentiating the tan Ψ spectrum, it is possible to estimate the effective wavelength change of optical interference caused by voltage application. Furthermore, a refractive index change corresponding to the effective wavelength change can be derived, and an effective electro-optic constant of the sample 100 that is an electro-optic thin film can be obtained from the refractive index change.
[0038]
Although the embodiment of the present invention has been described in detail above, the specific configuration is not limited to the present embodiment, and even if there is a design change or the like without departing from the gist of the present invention, Of course, it is included in the invention.
[0039]
【The invention's effect】
As described above, in the present invention, a significant intensity difference can be obtained spectrally by taking the difference spectrum of the tan Ψ spectrum when voltage is applied and when no voltage is applied. By comparing the amplitude of the difference spectrum with the amplitude of the reference spectrum obtained by differentiating the tan Ψ spectrum, it is possible to estimate the effective wavelength change of optical interference caused by voltage application. Then, the refractive index change corresponding to the effective wavelength change is derived, and the effective electro-optic constant of the electro-optic thin film as the sample can be obtained from this refractive index change.
[0040]
Therefore, the present invention is effective for evaluating the magnitude of the electro-optic effect of a thin film necessary for designing an optical waveguide or a functional optical component incorporating the electro-optic thin film.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing a configuration of an electro-optic constant measuring apparatus according to an embodiment.
FIG. 2 is an explanatory view for explaining a sample used in the electro-optic constant measuring apparatus of FIG.
FIG. 3 is a diagram showing a (tan Ψ, cos Δ) spectrum when an applied voltage is 0V.
FIG. 4 is a diagram showing a difference spectrum obtained by subtracting a tan Ψ spectrum at an applied voltage of 0 V from a tan Ψ spectrum at an applied voltage of 5 V, 10 V, and 15 V.
FIG. 5 is a diagram showing a [tan Ψ (x) −tan Ψ (x + 0.005 μm)] spectrum derived from the spectrum of FIG. 3;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Optical system 11 Adjustment stand 12 Light source part 12A Xenon lamp 12B, 12C Mirror 12D Polarizer 13 Light receiving part 13A Analyzer 13B Mirror 20 Spectrometer 21 Photomultiplier tube 30 Arithmetic unit

Claims (2)

Detection of (tan Ψ, cos Δ) by spectroscopic ellipsometry in a state where an electro-optic thin film formed on a substrate is covered with a transparent conductive thin film and a voltage can be applied between the substrate and the transparent conductive thin film. Measure the light wavelength dependent spectrum,
When a tan Ψ spectrum with and without voltage applied is obtained by measurement, the amplitude of the difference spectrum between the two tan Ψ spectra and the amplitude of the reference spectrum obtained by differentiating the tan Ψ spectrum with respect to the detection light wavelength are obtained. By comparing, calculate the effective wavelength change of optical interference caused by voltage application,
An electro-optic constant measuring method, wherein an electro-optic constant is calculated by deriving a change in refractive index from the value. Detection of (tan Ψ, cos Δ) by spectroscopic ellipsometry in a state where an electro-optic thin film formed on a substrate is covered with a transparent conductive thin film and a voltage can be applied between the substrate and the transparent conductive thin film. A measuring device (10, 20) for measuring a light wavelength dependent spectrum;
Obtained by differentiating the amplitude of the difference spectrum between the two tan Ψ spectra and the tan Ψ spectrum with respect to the detection light wavelength from the tan Ψ spectrum obtained when the voltage is applied and not applied obtained by the measuring device (10, 20). An arithmetic unit (30) that calculates the effective wavelength change of optical interference caused by voltage application by comparing the amplitude of the obtained reference spectrum, derives the refractive index change from the value, and calculates the electro-optic constant And an electro-optic constant measuring apparatus.

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