JPH08190008A - Grating element by surface acoustic wave and formation of grating element - Google Patents

Abstract

PURPOSE: To provide a grating element by surface acoustic waves of less higher harmonic noises by forming a thin film on the front surface of a substrate in such a manner that the film has optical characteristics having continuous changes and a formation method of the grating element. CONSTITUTION: This grating element has the substrate and the thin film formed on its front surface. The thin film has the optical diffraction characteristics of specified patterns and has the continuous shape changes in its front surface or has the continuous density changes within the thin film so as to have continuous refractive index changes. This method for forming the diffraction grating consists of stages for applying one to plural vibrations on the substrate from its one or multiple directions and uniformly forming the thin film on the front surface of the substrate in the state of generating standing waves in the front surface of the substrate. The surface acoustic waves are formed by vibration in the uniformly formed thin film, by which the thin film is changed to the physical state of the specified patterns having an optical diffraction effect. The thin film is maintained and immobilized in the changed state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave in which a substrate is vibrated at a constant period to form a standing wave in a thin film formed on the surface of the substrate, the surface having an optical diffraction characteristic. The present invention relates to a grid element and a grid element forming method.

[0002]

2. Description of the Related Art The phenomenon that light is diffracted by ultrasonic waves in liquids and solids is well known. This is because all optical materials change the material density when a force is applied, and the refractive index of light changes, which is called a photoelastic effect. When an ultrasonic wave is poured into a medium, a compression wave having a refractive index of light is generated by the photoelastic effect, and an element using the compression wave as a diffraction grating is called an acousto-optic effect element.

On the other hand, the same effect can be obtained by using a surface acoustic wave that propagates concentratedly near the surface of the medium. In particular, Y.Nakagawa, H.Ito &K.Kato; JJAP, Vol.32, p.4311
(1993) and Gunji, Hosaka, Kakio, Nakagawa; Acoustical Society of Japan Proceedings, as shown in _{2-1-13 (1994-3), Ta 2 O} 5 Lithium niobate (LiNbO ₃₎ substrate It has been revealed by the present inventor that when a surface acoustic wave is generated by applying ultrasonic waves to a thin film element formed by high-frequency sputtering the thin film of 1), the refractive index is improved by 1% and the photoelastic constant is increased by 3 to 6 times. It has become.

In the above invention, the diffracted light can be modulated by the high frequency power source provided outside the acousto-optic element and the surface acoustic wave element. That is, the intensity of the diffracted light can be modulated by modulating the intensity of the ultrasonic wave by the high frequency power source, and the diffracting direction of the diffracted light changes by changing the frequency of the ultrasonic wave.

On the other hand, optical components which produce a constant diffraction effect without change are roughly classified into a transmission type diffraction grating and a reflection type diffraction grating. The transmission type diffraction grating includes a transmission type amplitude grating and a transmission phase grating, and the reflection type diffraction grating includes a reflection type amplitude grating, a reflection type phase grating and a reflection type blazed grating. Explaining the typical structure of the diffraction grating as an example, the transmission type amplitude grating is formed by forming a thin pattern of thin metal or the like having a certain pattern with a certain pitch interval on the transmission substrate. . The transmissive phase grating is formed by forming thin grooves having a fixed pattern and a fixed pitch on a transmissive substrate. The reflection type amplitude grating is formed by forming a thin reflective metal film having a fixed pattern with a fixed pitch interval on a substrate that does not reflect light. Further, the reflection type phase grating is formed by forming a reflection metal plate on a substrate having grooves having a constant pattern with a constant pitch interval.

[0006]

With respect to the acousto-optic element and the surface acoustic wave element, when an ultrasonic wave is cut off, no diffraction effect occurs, so that a high frequency power source is constantly required. I have one. Therefore, for the purpose of obtaining a constant diffraction effect which does not need to be changed, the element is not generally used because it is very expensive.

With respect to the optical element for obtaining the above-mentioned constant diffraction effect, a very high precision manufacturing process is required to form a thin film at a constant interval or form a groove on a substrate. become. That is, since the wavelength of light is on the order of hundreds of micrometers, the position and direction of light interference change due to slight deviations in accuracy, making it impossible to satisfy the specifications of predetermined optical components. This lowers the specifications of the entire system that uses optical components, so that it is difficult to achieve a system with high accuracy, and even if it is achieved, there is a drawback that the cost becomes very high. Further, spatial harmonics are generated in the conventional diffraction grating, and even a diffraction grating for strengthening diffracted light of a specific order with respect to a specific wavelength shown in FIG. 11 as an example is theoretically The high-order diffraction of the _nth order is I _n
This occurs when / I ₀ = (2 / π) (1 / n ² ) and causes a problem of noise and spurious (unnecessary diffracted light).

As described above, the surface acoustic wave element is
The external high frequency power source and the vibration element propagate ultrasonic waves to the medium, thereby forming a sparse density on the surface, changing the light refractive index, and acting as a diffraction grating. Therefore, when ultrasonic waves are not incident, the medium is uniform and does not act as a diffraction grating.

Accordingly, the present invention provides a completely new grating element in which the action of the phase grating on the surface of the substrate brought about by the ultrasonic waves is maintained and fixed even when the ultrasonic waves are not incident, and a method of forming the same. It is provided.

Further, the present invention uses the above-mentioned surface acoustic wave to fix a surface refractive index which continuously changes with respect to the incident direction of ultrasonic waves, and a grating element having a completely new refractive index change, and a method of forming the same. To provide.

Further, the present invention is to provide a grating element having an arbitrary refractive index or the like by controlling the wavelength, amplitude and the like of the surface acoustic wave, and a method for forming the same.

In addition, it is another object of the present invention to provide a grating element which does not have the above-mentioned spatial harmonics, eliminates noise and spurious, and has highly accurate light diffraction, and a method for forming the grating element.

[0013]

Means for Solving the Problems When a surface acoustic wave is excited on a substrate in advance to generate a standing wave, and a thin film is formed by a high frequency sputtering method, the thin film formed is as shown in FIG. Thickness and refractive index are 1 of elastic surface wavelength
The present inventor's study revealed that a refractive index grating is formed by being perturbed in a pattern of / 2.

The grating element of the present invention focuses on the above-identified characteristics. The grating element has a substrate, a thin film formed on the surface thereof, and the thin film has a light diffraction characteristic of a fixed pattern. Is. Further, the thin film has a continuous shape change on the surface, or has a continuous refractive index change due to a continuous density change inside the thin film,
It is a lattice element.

Further, according to the present invention, a thin film is uniformly formed on the surface of the substrate in the state where a standing wave is generated on the surface of the substrate by applying one or a plurality of vibrations from one direction or multiple directions of the substrate, The uniformly formed thin film changes into a physical state of a certain pattern having a light diffraction effect by forming a surface acoustic wave by the vibration, and is maintained and fixed in the changed state. It is a method of forming a lattice element.

Furthermore, the present invention is a method for forming a grating element having desired optical characteristics by arbitrarily changing the amplitude and the vibration period of the vibration. That is, according to the present invention, since a grating can be formed with a period of 1/2 of the surface acoustic wave wavelength of a specific frequency, a grating having an arbitrary shape can be created and a diffraction grating having desired characteristics can be obtained. As a specific method, Fourier analysis is performed from the shape of the diffraction grating to determine and predict the spatial frequency component of the light obtained from the diffraction grating, and the diffraction grating to achieve the desired spatial frequency component is determined, and the elasticity is determined. The frequency and amplitude of the surface wave are determined, and one or more excitation electrodes are prepared for excitation. Therefore, it is also possible to form a diffraction grating element that selects a spatial frequency component and disperses a single light wave in two or more directions.

In the present specification, what is described as a substrate may be any one capable of forming a standing wave in a thin film, and may be in a film form, for example, a piezoelectric thin film in a film form. By making it, it becomes possible to excite surface acoustic waves.

The substrate and the thin film are not limited to those that reflect light, but may be those that transmit light. Therefore, the phase diffractive element or the amplitude diffractive element can be achieved by the combination of the transmission and the reflection.

[0019]

EXAMPLES Next, the constitution and effects of the present invention will be described in more detail with reference to examples, but these examples do not limit the present invention.

Example 1 A 128 ° Y-X crystal of lithium niobate (LiNbO ₃ ) was used as a substrate for surface acoustic wave (SAW) excitation.
A thin film of a ₂ O ₅ was formed by high frequency sputtering. Since Ta ₂ O ₅ is an insulating material having a high melting point, high frequency sputtering is suitable for forming a thin film.
Table 1 shows the sputtering conditions for forming the thin film.

[0021]

[Table 1] Sputtering conditions for thin film formation FIG. 2 shows an outline of the sputtering device used in the experiment. Here, a target of Ta is placed in Ar and O ₂ gas discharged at a high frequency of 13.56 MHz, and substrates having SAW standing waves are opposed to each other across a shutter. The board is fixed by the board holder shown in FIG.
Further, a comb-shaped electrode (IDT) for SAW excitation is provided. IDT has wavelength 20um, 30 pairs, intersection 2
mm. The high frequency for SAW excitation is 2 as shown in Table 1.
It is 00 MHz and 100 mW.

In order to evaluate the thin film thus obtained, the surface of the thin film was observed with a metallurgical microscope, and the state of the thin film was modeled (FIG. 4), followed by an optical probing method (FIG. 5).
The analysis was performed from the measurement by.

When observed with a metallurgical microscope, light and shade of light as shown in FIG. 6 was observed on the surface of the substrate. The pattern of this stripe was about 10 μm, which was ½ of the wavelength of SAW. Further, this fringe was not found at all in the area where SAW did not propagate. From this, it can be considered that these fringes are interference due to the influence of the unevenness of the thin film surface and the refractive index inside the thin film.

As an analytical model, based on the above observation, a model of the change of the film thickness and the change of the refractive index as shown in FIG. 4 was used. From the measurement of the optical probing method, the modeled change of the film thickness and the refractive index were obtained. Sought change.

In FIG. 4, A _{1 is the} incident light A ₂ is the reflected light that is reflected by the surface of the thin film and undergoes a phase change only from the ripples on the surface A ₅ : It is transmitted through the thin film, reflected by the surface of the substrate, and again outside the thin film come out a reflected light, the ripple with the interior of the refractive index reflected light undergoes a phase change from changes in the thin film surface, and then, the incidence angle phi = 45 DEG of a _1, the refractive index of air n _a = 1, Ta ₂ O Assuming that the refractive index of ₅ is n _O = 2.11, and the refractive index of Nb ₂ O ₅ is n _s = 2.23, then | A ₂ | / | A ₁ | = 0.2279 | A ₅ | / | A ₁ | = 0.0275, and multiple reflections in the film are neglected.

When the surface of the thin film arranged as shown in FIG. 5 is irradiated with laser light, Raman-Nass scattering occurs due to the periodicity of the film thickness. When the diffraction intensity is calculated from the model of FIG.
The order diffracted light intensity as _{I n, I 1 / I 0} = {A 2 (W 1/2) + A 5 (W 2/2)} 2 /
(A ₂ + A ₅ ) ² is obtained (provided that A ₁ , A ₂ , and A ₅ are the maximum amplitudes of the respective lights). Here, W ₂ = (2π / λ) · 2Δh · cosφ W ₅ = (2π / λ) · 2hΔn / cos θ + (2π /
λ) · 2Δh (n _O / cosφ _O −cosφ).

Further, when aluminum is vapor-deposited on the surface of the thin film and totally reflected on the surface of the thin film, the relative amplitude is A ₁ =
Since 1, A ₂ = 1 and A ₅ = 0, the effect of only the ripple on the surface can be seen.

FIG. 7 shows the measurement result of the diffracted light intensity. The thin film was sputtered for 20 minutes under the conditions shown in Table 1, and the film thickness was about 20 nm. The horizontal axis of FIG. 7 is the distance from the center of the IDT, and it can be seen that when the width of the IDT deviates, almost no diffraction occurs. From this result, Δh / h = 1.
78%. The diffraction intensity when the aluminum film is not provided on the surface is 216 times that when there is aluminum. Using the value of Δh / h above, from the above equation, Δn / n _O
= 5.89%.

As described above, it was confirmed that a thin film having a structure in which the film thickness and the refractive index are constant can be formed by the above film forming method according to the standing wave of the surface acoustic wave.

That is, regarding the method of forming the lattice element, when a standing wave of a surface acoustic wave is excited on the substrate by the applied vibration, a thin film formed by irradiation by sputtering or the like, or a thin film formed in advance and still fixed. Among the thin films that are not formed, the thin film in the streak part of the standing wave does not receive any amplitude energy while remaining in that part because there is no amplitude, whereas the film formation in the antinode part of the standing wave is
The amplitude energy receives a large amount of energy, and the conditions for film formation on the abdomen and muscles change, causing changes in the light diffraction characteristics of a certain pattern, that is, changes in the refractive index of the medium and changes in the surface film thickness. Become. As a result, it is possible to obtain a lattice element having a fixed pattern.

Although the example shown in the above measurement describes a reflection type diffraction grating when it is reflected on the surface of a thin film, it can be a transmission type diffraction grating when the material of the thin film is light transmissive. At the same time, it is clear that the present invention functions as an amplitude diffraction grating or a phase diffraction grating. This also applies to the examples described below.

Specific examples that will be realized by the present invention will be described below.

As Example 1, vibration is applied from one direction,
It is possible to achieve the one-dimensional lattice formation method by exciting and propagating the surface acoustic wave to generate a reflected wave from the opposite direction (end face) and a one-dimensional standing wave.

As a second embodiment, vibration can be applied from both opposite directions, and surface acoustic waves can be excited and propagated from both directions to generate a one-dimensional standing wave, thereby achieving a one-dimensional lattice formation method. . As a result, a larger and ideal standing wave can be created as compared with the first embodiment. As Example 3, vibration is applied from two directions, and a two-dimensional standing wave can be generated by a reflected wave from each end face, and a two-dimensional lattice forming method can be achieved. Further, various two-dimensional lattices can be created by arbitrarily changing the intersecting angles of the two directions.

As a fourth embodiment, it is possible to generate a two-dimensional standing wave by applying vibrations from four opposite directions (FIG. 8), and to achieve a two-dimensional lattice forming method. In Figure 8, it is an oscillating wave traveling at an angle of 90 °,
By changing this angle arbitrarily, various two-dimensional lattices can be created.

As a fifth embodiment, it is possible to apply a vibration from a ring-shaped oscillator to form a ring-shaped standing wave (FIG. 9) and form a ring-shaped grating in the two-dimensional grating forming method. it can.

As a sixth embodiment, a lattice element having desired optical characteristics can be formed by changing the amplitude or period of the acoustic vibration to be applied, or the amplitude and the period at the same time.

As a seventh embodiment, a metal or a dielectric material or the like is sputter-irradiated or vapor-deposited while acoustic waves are applied to generate a standing wave on the substrate to form a thin film having an arbitrary thickness. The thin film changes into a physical and shape thin film having predetermined optical characteristics by the surface acoustic wave. Even when the acoustic vibration is stopped, the dielectric material or the like is fixed as a physical or shape thin film in a state where the optical characteristics are maintained, so that a predetermined lattice element can be formed.

As Example 8, the exposure developer is applied on the substrate in advance, or the exposure developer is applied on the surface of the substrate during the exposure to form a thin film having an arbitrary thickness. The thin film changes into a physical and shape thin film having predetermined optical characteristics by the surface acoustic wave. After that, by curing with a thin film curing means (however, it may be soft curing or hard curing), the physical and shape of the state where the optical characteristics are maintained even if the acoustic vibration is stopped. Since the thin film is fixed as it is, a predetermined lattice element can be formed. Whether or not the film is completely fixed thereafter depends on the process and the material, but is not essential to the present invention.

As a ninth embodiment, in the above-mentioned seventh embodiment,
A mask is arranged between the sputtering irradiation unit or the vapor deposition generation unit and the substrate to be irradiated. Since the mask has a pattern, a thin film having a pattern corresponding to the pattern of the mask is formed on the surface of the substrate by sputtering irradiation or vapor deposition. This allows
A grating element having a diffraction effect can be formed only on the formed portion.

As a tenth embodiment, in the eighth embodiment, a mask is arranged between the vapor deposition source part and the substrate to be vapor deposited. Since the mask has a pattern formed thereon, the exposed portion becomes a thin film having different developing characteristics from the unexposed portion on the surface of the substrate, corresponding to the pattern of the mask. As a result, it is possible to form the grating element having the diffraction effect only in the patterned portion by performing the development thereafter.

It will be apparent that the exposure step can be accomplished both in the negative type and the positive type. Further, there is no essential difference in whether the exposure is performed before curing or after curing.

As Example 11, a perspective view of a Bragg diffraction type optical modulator device structure is shown in FIG. 10 as an example of using the grating device of the present invention as a waveguide.

The incident light entering the prism from the left side of FIG. 10, enters the thin Ta ₂ 0 ₅ through the prism, the process proceeds to the right of the prism direction the thin film as a waveguide. At this time, since the thin film is the grating element of the present invention, the light propagating in the waveguide is affected by the change in the lattice density in the thin film to cause so-called Bragg diffraction and has a certain deviation angle with the incident direction. I will have it. The light having a declination passes through the waveguide and exits as diffracted light via the prism at the right end. The light shown as the emitted light on the right side is the emitted light when the waveguide is not the grating element of the present invention but a simple waveguide, and it is clear that the emitted light and the diffracted light have a spectral angle. This is for reference only. Therefore, it is clear that the grating element of the present invention can be used as an optical element used as a waveguide,
By using it as a Bragg diffraction element, the reflection efficiency can be easily made 100%.

[0045]

According to the present invention, since the optical characteristics of the diffraction grating can be controlled by the frequency and the amplitude of the applied vibration, it is possible to provide an optical element that flexibly meets the requirements.

The optical characteristics are due to a physical standing wave. Since the standing wave depends on physical characteristics such as the elastic coefficient of the medium, a very stable standing wave is formed. It is possible. Therefore, it is possible to provide a simple and stable precision optical element as compared with an optical element which is extremely carefully manufactured by conventional precision engineering.

Further, since the change of the thin film surface of the present invention can be made into a very clean sine wave shape, spatial harmonics can be eliminated, and noise and spurious are not generated.

Further, as shown in the above embodiment, according to the present invention, it is possible to obtain a one-dimensional, two-dimensional and ring-shaped phase grating having an arbitrary pattern, and an optical demultiplexer having an arbitrary wavelength is constructed. it can. Further, according to the method of the present invention, a superlattice crystal of a semiconductor thin film can be prepared. Therefore, the function of the optical communication device can be enhanced by using the diffraction grating thin film according to the present invention. Furthermore, by using the thin film as a mold, it is possible to transfer an arbitrary number of pattern structures of the thin film. As a result, not only the mass production of the above optical communication device but also the production of a film having a decorative effect such as a film having a special reflection characteristic (called a hologram paper) is possible.

[Brief description of drawings]

FIG. 1 is a schematic view of a grating element using surface acoustic waves.

FIG. 2 High-frequency sputtering device (manufactured by Nichiden Anelva,
It is a schematic diagram of a SPF-210A sputtering device).

FIG. 3 is an example of a substrate holder for making a one-dimensional thin film grating.

FIG. 4 is a schematic plan view in which a thin film of a one-dimensional lattice is modeled.

FIG. 5 is a schematic diagram of an optical probing method used to measure a change in film thickness and a change in refractive index of a thin film.

FIG. 6 is a metallographic observation result of the surface of the thin film obtained in the present invention.

FIG. 7 is an example of the measurement result of the diffracted light intensity of the thin film obtained in the present invention.

FIG. 8 is a schematic diagram for achieving the grid-shaped two-dimensional grid forming method shown in the fourth embodiment.

FIG. 9 is a schematic diagram for achieving the circular two-dimensional lattice forming method shown in the fifth embodiment.

FIG. 10 is a perspective view of the waveguide type Bragg diffraction type optical modulator device structure shown in Example 11;

FIG. 11 is a cross-sectional view of a prior art reflective diffraction grating with a discontinuous surface.

Claims (14)

[Claims] 1. A lattice element using surface acoustic waves, comprising a substrate and a thin film formed on the substrate, the thin film having an optical diffraction characteristic that changes in a fixed pattern in the plane direction of the thin film. 2. The surface acoustic wave grating element according to claim 1, wherein the constant-period optical diffraction characteristics are provided by changing the shape of the thin film on the surface in a constant pattern. 3. The surface acoustic wave grating element according to claim 1, wherein the constant-period optical diffraction characteristic is brought about by a change in the refractive index inside the thin film in a constant pattern. 4. The lattice element according to claim 1, wherein the optical diffraction characteristic of the constant period is brought about by changing the shape on the surface of the thin film and the inside of the thin film in a constant pattern. 5. To apply a standing wave to the surface of the substrate, one or more vibrations are applied from one or multiple directions of the substrate to form a thin film on the surface of the substrate, and the vibration causes the thin film to generate a surface acoustic wave. A method of forming a grating element by a surface acoustic wave, comprising forming a standing wave, the thin film having a constant physical characteristic pattern by the standing wave, and fixing the pattern to the characteristic. 6. A lattice element formation by a surface acoustic wave according to claim 5, wherein a lattice element having arbitrary optical characteristics is formed by simultaneously changing the amplitude or the period of the applied vibration, or the amplitude and the period at the same time. Method. 7. The method of forming a lattice element by surface acoustic wave according to claim 5, wherein the thin film is formed on the surface of the substrate by sputtering irradiation. 8. The method for forming a lattice element by surface acoustic wave according to claim 5, wherein the thin film is formed on the surface of the substrate by vapor deposition. 9. The method of forming a lattice element by surface acoustic wave according to claim 5, wherein the thin film is formed on the surface of the substrate by applying an exposure developer. 10. In order to apply a standing wave to the surface of the substrate, one or a plurality of vibrations are applied from one or multiple directions of the substrate, and a mask is provided between the sputtering irradiation unit or the evaporation source unit and the irradiation target or the evaporation target substrate. The mask is patterned, and a thin film having a pattern corresponding to the pattern of the mask is formed on the substrate surface by sputtering irradiation or vapor deposition, and the vibration causes the thin film to have an elastic surface. A standing wave by a wave is formed, the thin film has a certain physical characteristic pattern by the standing wave, and is fixed to the characteristic, and only the deposited portion has a diffraction effect. A method for forming a grating element by using a surface acoustic wave. 11. A grating element formation by a surface acoustic wave according to claim 10, wherein a grating element having arbitrary optical characteristics is formed by simultaneously changing the amplitude or the cycle, or the amplitude and the cycle of the applied vibration. Method. 12. In order to apply a standing wave to the surface of the substrate, one or a plurality of vibrations are applied from one or multiple directions of the substrate, an exposure photosensitizer is applied to the substrate, and the exposure irradiation unit and the substrate to be irradiated are exposed. A mask is arranged, a pattern is formed on the mask, the thin film is exposed by a pattern corresponding to the pattern of the mask, the vibration causes the thin film to form a standing wave by a surface acoustic wave, The thin film has a fixed physical characteristic pattern by the standing wave and is fixed to the characteristic, and the substrate surface having the thin film is developed, and only the portion where the thin film remains has a diffraction effect. A method of forming a lattice element using surface acoustic waves, the method comprising: 13. A lattice element formation by a surface acoustic wave according to claim 12, wherein a lattice element having arbitrary optical characteristics is formed by simultaneously changing the amplitude or the period of the applied vibration, or the amplitude and the period at the same time. Method. 14. An optical element in which the thin film of the grating element according to claim 1 is a light waveguide.