JPH0711609B2 - External diffusion thin film optical waveguide and method of manufacturing the same - Google Patents

Description

The present invention relates to an external diffusion thin film optical waveguide such as an integrated optical structure and a method for manufacturing the same.

Currently, in the case of realizing an optical deflector and an optical modulator by an integrated optical structure, lithium niobate (hereinafter referred to as LiNbO), which is excellent in piezoelectricity, photoacoustic effect, and electro-optical effect as an optical waveguide substrate and has a small optical propagation loss. ₃ ) and lithium tantalate (hereinafter referred to as LiTaO ₃ ) crystals are widely used.

As a typical method for producing a thin film optical waveguide using the crystal substrate, a titanium (hereinafter referred to as Ti) metal is thermally diffused on the surface of the crystal substrate at a high temperature to form a film on the surface of the crystal substrate. There is a method of forming an optical waveguide layer having a refractive index slightly higher than that of the substrate. However, the thin-film optical waveguide manufactured by this method is susceptible to optical damage, and has a drawback that only light of very low power can be introduced into the waveguide. The term "optical damage" as used herein means that "when the intensity of light input to an optical waveguide is increased, the intensity of light propagating in the optical waveguide and extracted to the outside is proportional to the input intensity due to scattering. Phenomenon that does not increase. "

As a method for producing an optical waveguide for improving the optical damage,
A LiNbO ₃ or LiTaO ₃ crystal substrate is heat-treated at a high temperature, lithium oxide (hereinafter referred to as Li ₂ O) is diffused out of the crystal substrate, and lithium having a slightly larger refractive index than the substrate (hereinafter referred to as Li ₂ O) There is a method of forming a vacancy layer. By the Li ₂ O outdiffusion method, the threshold of optical damage is higher than the internal diffusion process of Ti metal literature [R.
L. Holman & P.J. Cressman, IOC, 90, 28 April (1981)].

By the way, when the optical deflector and the optical modulator are to be realized by utilizing the photoacoustic effect and the electro-optical effect, it is important to increase the efficiency of each of the above effects in the formation of the element. As a typical example of utilizing the photoacoustic effect, there is a method of applying high-frequency electrolysis to a comb-shaped electrode manufactured by photolithography on an optical waveguide to cause surface acoustic waves to swell on the optical waveguide.
In this case, it is known that the interaction between the surface acoustic wave excited on the optical waveguide and the guided light propagating in the optical waveguide increases as the energy distribution of the guided light is confined near the substrate surface. (CSTsai, IEEE TRANSACTIONS
ON CIRCUITS AND SYSTEMS, VOL.CAS-26,12,1979] From the viewpoint of maximizing the use of the above interaction, the optical waveguide (Li vacancy layer) formed by the Li ₂ O external diffusion method described above is used. The thickness is 10 to 10 because the change in the refractive index is small.
It is necessary to make the thickness as thick as 0 μm, which is not preferable because the energy distribution of the guided light spreads in the thickness direction. Therefore, when the thin film optical waveguide manufactured by the above-mentioned Li ₂ O external diffusion method is used for the optical deflector or the like, it is difficult to realize a highly efficient device.

In addition, the Li ₂ O outdiffusion method has a problem that a channel type waveguide cannot be formed because it is performed only by heat treatment.

On the other hand, an ion exchange method is known as another method for producing a thin film optical waveguide that improves optical damage. This method uses thallium nitrate (hereinafter referred to as TlNO ₃ ) or silver nitrate (hereinafter AgNO ₃
Lithium ion in the crystal substrate within a range of about 1 to 3 μm in the depth direction from the surface of the crystal substrate by low-temperature heat treating the crystal substrate of LiNbO ₃ or LiTaO ₃ in a solvent salt of (Li ⁺ ) is exchanged with ion species of thallium (Tl) or silver (Ag) in the molten salt, and a large difference in refractive index (△
An optical waveguide layer having a thickness of n to 0.12) is formed.

However, the threshold of optical damage of the thin film optical waveguide prepared by the above ion exchange method has been reported in the literature [JL Jackel, DHOls
on and AMGlass, J.Appl.Phys.52 (7) July 1981], it is not enough just to improve by about twice as much as the Ti internal diffusion method, and the processing time is about It had the problem of being as long as 10 hours.

An object of the present invention is to solve the above-mentioned problems of the conventional example, a threshold value of optical damage is sufficiently high, and an external diffusion thin film optical waveguide having a large interaction between guided light and a photoacoustic effect or an electro-optical effect, and It is to provide a manufacturing method thereof.

Another object of the invention is that the threshold of optical damage is high enough,
Another object of the present invention is to provide a channel type external diffusion thin film optical waveguide in which the interaction between guided light and photoacoustic effect or electro-optical effect is large, and a method for manufacturing the same.

Another object of the invention is that the threshold of optical damage is high enough,
An object of the present invention is to provide an external diffusion thin film waveguide and a method for manufacturing the same, which requires a short processing time during manufacturing.

The above object of the present invention is to provide an external diffusion thin film optical waveguide having a lithium vacancy layer formed by externally diffusing lithium oxide in the crystal on the surface of a substrate made of a lithium niobate crystal or a lithium tantalate crystal. An external diffusion thin film optical waveguide having an ion species injection layer having a higher refractive index than the lithium vacancy layer formed by further injecting an ion species near the surface of the lithium vacancy layer. And an external diffusion thin film optical waveguide including a process of externally diffusing lithium oxide in the crystal by heat-treating the surface of a substrate made of a lithium niobate crystal or a lithium tantalate crystal at a high temperature. In the method for manufacturing the same, after forming the lithium vacancy layer, an ion is further formed near the surface of the lithium vacancy layer. It is achieved by a method for manufacturing a outdiffusion thin film optical waveguide and having a process of forming an ion species implanted layer of high refractive index than the lithium vacancy layer by implanting species.

The present invention will be described below with reference to the drawings.

FIG. 1 is a schematic view showing the structure of an external diffusion thin film optical waveguide of the present invention. In the figure, 1 is a crystal substrate, 2 is a Li vacancy layer,
Reference numeral 3 indicates an ion species injection layer. Here, as a material of the crystal substrate 1, X-cut [the substrate surface is perpendicular to the crystal axis direction (2,0) or (, 1,0)] LiNbO ₃ , Z cut [the substrate surface is the crystal axis direction (0, Perpendicular to 0,1)] LiNbO ₃ , X cut LiTaO ₃ , Z
It is possible to use either crystal of cut LiTaO ₃ . The Li vacancy layer is formed by heating the crystal substrate at a high temperature to obtain Li ₂ O.
Are formed by external diffusion. The ionic species implantation layer 3 can be formed by implanting ionic species into the Li vacancy layer 2 by using an ion exchange method or the like. In the external diffusion thin film optical waveguide of the present invention, Li
The amount of change in the refractive index of the vacancy layer 2 relative to the crystal substrate 1 is 0.
The amount of change in the refractive index of the ion species injection layer 3 is about 0.1, and the guided light propagates in the ion species injection layer 3. Further, the Li vacancy layer 2 below the ionic species injection layer 3 captures photoelectrons generated by light irradiation by the Li vacancy,
It has an effect of preventing the optical waveguide from being damaged by the photoelectrons and improving the threshold value of the optical damage. In order to make full use of this effect, it is desirable that the Li lattice layer 2 has a thickness of 10 to 100 μm in consideration of the depth of guided light penetrating into the crystal substrate.

Next, in the present invention, the mechanism in the case of implanting ion species into the LiNbO ₃ crystal substrate by using the ion exchange method will be described. The LiNbO ₃ crystal is a trigonal crystal as shown in FIG.
Lithium (Li) 5, holes 6, and niobium (Nb) 7 are sequentially arranged on the shaft 8. Here, 4 is oxygen (O).

When the LiNbO ₃ crystal shown in FIG. 2 (a) is heat-treated at a high temperature of about 1000 ° C., Li ions are released to the outside to form Li vacancies 9 as shown in FIG. 2 (b). In the above state, when ion species such as Ag, Tl, and K are implanted by, for example, an ion exchange method, the ion species 10 is present at the position of the Li vacancies as shown in FIG. 2 (c). In a normal ion exchange mechanism, Li ions and implanted ions are exchanged at the same time, resulting in a long processing time. On the other hand, in the case of the present invention, since the Li site is externally diffused by the heat treatment and is vacant before the ion exchange treatment, the ion species are easily injected, and the treatment time can be shortened as compared with the usual ion exchange method. Further, an optical waveguide having a larger difference in refractive index can be realized.

Further, since the optical waveguide layer formed of the ion species injection layer of the present invention has a large difference in refractive index as described above, it is possible to reduce the thickness of the optical waveguide layer.

In FIG. 3, 11 is the amplitude distribution of the surface acoustic wave at a frequency of 1 GHz in the depth direction of the crystal substrate, 12 is the energy distribution of the guided light in the external diffusion thin film optical waveguide of the present invention, and 13 is the conventional Ti.
6 shows the Energer distribution of guided light in an optical waveguide manufactured by internal diffusion. As shown in FIG. 3, the surface acoustic wave intensity has a peak about 0.5 μm from the substrate surface in the depth direction. On the other hand, the peak of the energy distribution 12 of the optical waveguide of the present invention is also about 0.7 in the depth direction from the surface.
μm exists everywhere, and the energy distribution of surface acoustic waves
The overlapping ratio with 11 becomes dramatically larger than the conventional one, and the interaction between the guided light and the surface acoustic wave increases. In the present invention, in order to obtain a large interaction as described above,
The optical waveguide layer, that is, the ion species injection layer is as thin as possible,
About μm is desirable.

Hereinafter, the present invention will be specifically described with reference to examples.

Example 1. An x-cut LiNbO ₃ crystal substrate (2 mm thick in the x direction, 1 inch in each of the z direction and the y direction) was used to make one surface (for example, the x ⁺ surface) flat within a few Newton rings. After polishing,
Usual ultrasonic cleaning with methanol, acetone and pure water was performed, and nitrogen gas was blown to dry it. Next, the LiNb
In order to externally diffuse LiO ₂ from the O ₃ substrate, the washed and dried substrate was placed in a holder made of fused silica and set in a thermal diffusion furnace at 1000 ° C. ₂ dry O ₂ gas as atmosphere gas
It was introduced into the diffusion furnace at a flow rate of / min. From room temperature to 1000 ° C
A second thermal diffusion in which the temperature inside the furnace was raised at a rate of 16 ° C / min, and after 1 hour the temperature inside the furnace became constant, then the temperature was kept at 1000 ° C for 8 hours, and then the substrate was kept at 600 ° C. Moved to the furnace.
Further, the power supply to the second diffusion furnace was stopped and the temperature was allowed to cool from 600 ° C. to room temperature. The reason for quenching the substrate from 1000 ° C to 600 ° C is to prevent LiNb ₃ O _{8 from} being formed in the substrate and form an optical waveguide with little optical propagation loss according to the example.
If a light propagation loss of about 1.5 to 1 dB / cm can be tolerated,
It may be formed by cooling from 1000 ° C. using a single heat diffusion path.

In order to measure the characteristics of the LiO ₂ external diffusion optical waveguide obtained after cooling, the He-Ne light with a wavelength of 6328 Å was introduced from the rutile prism and the optical propagation loss was measured to produce a good optical waveguide of 0.3 dB / cm. It turned out that it was done. This is 200Å
The optical propagation loss was almost the same as the optical waveguide fabricated by internally diffusing Ti to the thickness of. When the propagation mode was measured by changing the incident angle to the prism, four modes were observed in TE mode. When the input light beam diameter was set to 2 mm and the ratio of the input light to the output light was changed to change the input light intensity, the optical damage was measured and the optical waveguide due to Ti internal diffusion was damaged within 1 second with the input light of 1 mW. In contrast, the LiO ₂ outdiffusion resulted in a stable output value for several hours even with an input light of 20 mW. From this, LiO ₂ outdiffusion
It was found that an optical waveguide with an optical propagation loss equal to that of an optical waveguide fabricated by Ti internal diffusion, and an optical waveguide having a damage threshold of about 20 times can be fabricated. When the LiO ₂ outdiffusion depth was calculated from the mode index, it was about 10
μm, the refractive index difference Δn from the substrate was 0.01, and a thick Li vacancy layer was formed.

After measuring the optical characteristics, the above-mentioned cleaning of the substrate was repeated again to clean the substrate surface, and then the ion exchange treatment was carried out. Ion exchange treatment was carried out by placing 30 g of special grade AgNO ₃ crystal pieces in a 100 CC quartz beaker and washing and drying the same.
The substrate after Li ₂ O outdiffusion is placed with the surface on which the Li vacancy layer is formed facing up, and the beaker is placed in a heating furnace at a temperature of 330 ° C.
Held for hours. It is desirable to stir the molten salt during ion exchange. At this time, attention was paid so that the NO ₂ gas generated from the AgNO ₃ solution could be discharged to the outside of the furnace. After such an ion exchange treatment, the furnace was de-energized and left to stand until the temperature in the furnace returned to room temperature. After that, the solidified substrate and AgNO ₃ were taken out from the heat furnace with the beaker taken out, and pure water warmed to about 60 to 70 ° C. was poured into the beaker and allowed to stand until the substrate and AgNO ₃ were separated. After 1 hour, only the separated substrate was taken out, and further washed with warm water for several hours until the AgNO ₃ microcrystals were completely removed from the substrate surface. When it is difficult to remove the fine crystals, the surface of the optical waveguide is lightly polished by using an abrasive such as colloidal silica.

In order to investigate the characteristics of the planar type external diffusion thin film optical waveguide of the present invention produced by him, the wavelength of 6328 was measured by a rutile prism.
The He-Ne light of Å was again introduced into the waveguide surface in the y direction, and the optical propagation loss was measured to obtain a value of 0.6 dB / cm. The optical damage threshold of He-Ne light was 24 mW / cm. X for comparison
The optical propagation loss is 1 dB / cm when only ion exchange is performed under the same conditions without external diffusion of LiO ₂ on the cut LiNbO ₃ substrate.
The optical damage threshold was 4 mW / cm.

The propagation mode of the external diffusion thin film waveguide prepared in this example was almost the same as the propagation mode TEo of the optical waveguide prepared only by the ion exchange method under the same conditions.

Next, the optical waveguide of the present example after the measurement was washed again and dried, and then the positive photoresist was spinner-coated to a thickness of 1 to 1.5 μm.
m was spinner-coated, and contact exposure was performed with a negative mask of a comb-shaped electrode, and a phenomenon occurred so that only the comb-shaped electrode portion was not left.
After washing with water, drying, loading in a vacuum evaporation system, and 1 × 10 ^-6 Torr
Evacuation was performed and Al (film thickness 1500Å) was deposited by EB deposition. The Al film on the photoresist was removed by lift-off by immersing in acetone for several minutes after vapor deposition, and only the comb-shaped electrode portion was formed on the optical waveguide. At this time, the comb-shaped electrode was designed so that the center wavelength of the surface acoustic wave was 600 MHz. Therefore, both the electrode width and the electrode interval of the comb-shaped electrode were
It was 1.45 μm. Thus, a comb-shaped electrode was provided on the optical waveguide of the present invention to fabricate an optical waveguide type optical deflector.

On the other hand, with respect to the optical waveguide produced by thermally diffusing Ti to a thickness of 200Å, a comb-shaped electrode is formed by the lift-off method similar to the above to produce an optical wave type optical deflector, The performance of the optical waveguide type optical deflector made by using the optical waveguide was compared under the following conditions. In both cases, the incident light uses a He-Ne laser with a wavelength of 6328Å, and 0.6 W is applied to the comb electrode.
RF power was applied. When the RF frequency is 600 MHz, the diffraction efficiency is about three times as high as that produced on the optical waveguide of this embodiment, as compared with the optical waveguide type optical deflector produced on the Ti thermal diffusion optical waveguide. Diffraction efficiency was obtained. The intensity of He-Ne laser light was measured at 1 mW or less.
When the laser light intensity was increased to W, 15 mW, in the case of the Ti thermal diffusion optical waveguide, the input intensity was 5 mW, optical damage had already occurred, and the output light was reduced catastrophically, but in the case of this example, the input intensity was
It was found that the output light intensity increased in proportion to the input light intensity even when it exceeded 15 mW, and no optical damage occurred.

Second Embodiment Next, a second embodiment of the present invention will be described. In the case of the first embodiment, the ion exchange treatment was carried out in 330 ° C. AgNO ₃ molten salt, but in the second embodiment, the ion species is K (potassium), so the ion exchange treatment is carried out in KNO ₃ molten salt. I went to.
The x-cut LiNbO ₃ substrate after LiO ₂ outdiffusion prepared under the same conditions as in the first embodiment was treated in a 360 ° C. KNO ₃ molten salt for several tens of minutes to several hours to fabricate the optical waveguide of the present invention.
An optical waveguide with a low optical propagation loss of 0.48 dB / cm could be fabricated. A comb-shaped electrode was formed on the optical waveguide in the same process as in the first embodiment, and RF power of frequency 600 MHz and 0.6 W was applied to the comb-shaped electrode, and the diffraction efficiency was measured. The almost same result was obtained. Also, with respect to optical damage, the same results as in the first embodiment were obtained. KNO ₃ has a higher melting point than AgNO _{3 and} has a low rate of decomposing into other compounds even in a heated state of about 360 ° C. to 400 ° C., so that light propagation loss is smaller than when AgNO ₃ molten salt is used, and The advantage is that a waveguide with a large difference in refractive index can be manufactured.

[Embodiment 3] Next, a case of a channel type external diffusion thin film optical waveguide according to a third embodiment of the present invention will be described. First, LiO ₂ outdiffusion treatment is performed under the same conditions as in the first embodiment, and then the LiO ₂ outdiffusion x-cut LiNbO ₃ substrate is washed again and dried, and then a negative photoresist is spun to a thickness of 1 to 1.5 μm. Was spinner-coated, and contact exposure was performed with a negative mask of a channel type optical waveguide, and development was performed so that only the channel portion remained. After washing with water, drying, loading in a vacuum evaporation system, evacuation to 1 x 10 ^-6 Torr, and EB evaporation, the metal film Al (film thickness 1500
Å) was deposited. However, the metal film is not limited to Al, and may be a metal film (for example, Ti, Au) that prevents ionic species from entering the substrate in the subsequent ion exchange treatment.

The Al film deposited on the photoresist was removed by lift-off by immersing it in acetone for several minutes after vapor deposition, and the Al film was formed only on the portion other than the channel portion. At this time, the channel width was 5 μm.

Next, the same ion exchange treatment as in the first embodiment was performed in order to form a layer in which the ion species were implanted in the above channel portion. After the above ion exchange treatment, the metal film was covered with an etching solution and rewashed, whereby a channel type optical waveguide having the structure of the present invention could be manufactured. Here, when the metal film is Al, phosphoric acid may be used as the etching liquid. The light propagation loss and the optical damage were measured in the same manner as in the first embodiment, but almost the same results were obtained.

Fourth Embodiment Next, a fourth embodiment of the present invention will be described.

The above-described embodiment includes a wet treatment process called ion exchange treatment, but the fourth embodiment is one in which the optical waveguide of the present invention is manufactured by a dry treatment process. First, a LiNbO ₃ crystal substrate is subjected to LiO ₂ external diffusion treatment under the same conditions as in the first embodiment. Subsequently, the above substrate was loaded into a vacuum vapor deposition apparatus and 1 × 10 ⁻⁶ To
Evacuate to rr and deposit a metal film such as
Ag (film thickness 2000Å) was vapor deposited. Put it in the electric furnace again, 550
Heat treatment was performed at 1 ° C. for 1 hour. It takes 0.5 hours to rise to 550 ° C, and after heat treatment, stop energizing and
Heat was released from ℃ to room temperature. By the above heat treatment,
The ionic species in the metal film diffused into the substrate, and the optical waveguide of the present invention could be manufactured. After the fabrication, the light propagation loss and the optical damage were measured in the same manner as in the first embodiment, and almost the same results as in the first embodiment were obtained. In the present embodiment, the metal to be internally diffused is not limited to Ag, and other metals (such as Ti) may be used.

In the above-mentioned embodiments, an example in which LiNbO ₃ is used as the crystal substrate has been shown, but the present invention can be carried out in the same manner even if a LiTaO ₃ crystal substrate is used.

As described above, according to the present invention, in the conventional thin film optical waveguide and the manufacturing method thereof, 1) increase the threshold value of optical damage, 2) when it is used for an optical deflector or the like, guided light and photoacoustic effect and electrical It has the effect of shortening the processing time required for increasing the interaction with the optical effect 3).

[Brief description of drawings]

FIG. 1 is a schematic view showing the structure of an external diffusion thin film optical waveguide of the present invention, and FIGS. 2 (a), (b) and (c) respectively explain the state of ion species implantation in the manufacturing process of the present invention. FIG. 3 is a schematic diagram showing the energy distribution of guided light and the amplitude distribution of surface acoustic waves in the depth direction of the substrate in the present invention. 1 ... Crystal substrate, 2 ... Li vacancy layer, 3 ... Ion seed injection layer, 4 ... Oxygen, 5 ... Lithium, 6 ... Vacancy, 7 ...
... niobium, 8 ... C axis, 9 ... Li vacancy, 10 ... ion species.

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Claims (2)

[Claims] 1. An external diffusion thin film optical waveguide having a lithium vacancy layer formed by externally diffusing lithium oxide in the crystal on a surface of a substrate made of a lithium niobate crystal or a lithium tantalate crystal, An external diffusion thin film optical waveguide comprising an ion species injection layer having a higher refractive index than the lithium vacancy layer formed by further implanting an ion species near the surface of the lithium vacancy layer. 2. External diffusion having a process of externally diffusing lithium oxide in the crystal by heat-treating the surface of a substrate made of a lithium niobate crystal or a lithium tantalate crystal at a high temperature to form a lithium vacancy layer. In the method for manufacturing a thin film optical waveguide, after forming the lithium vacancy layer, an ionic species is further injected near the surface of the lithium vacancy layer to form an ionic species injection layer having a higher refractive index than the lithium vacancy layer. A method of manufacturing an external diffusion thin film optical waveguide, which comprises the steps of: