JPS5991403A - Thin film optical waveguide for external diffusion and its manufacture - Google Patents

Abstract

PURPOSE:To improve the threshold value against an optical damage by forming an ion seed implanted layer and a lithium vacancy layer on the surface of an LiNbO3 crystal substrate or LiTaO3 crystal substrate. CONSTITUTION:An Li vacancy layer 2 is formed by subjecting a lithium niobate LiNbO3 crystal substrate or lithium tantalate LiTaO3 crystal substrate to a high temp. heat treatment and diffusing externally Li2O. An ion seed implanted layer 3 is formed by implanting an ion seed by using an ion exchange method. The layer 2 captures the photoelectron generated by irradiation of light by Li vacancy, thereby preventing the damage of an optical waveguide by photoelectron and improving the threshold value against an optical damage.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an externally diffusing thin film optical waveguide, such as an integrated optical structure, and a method for making the same.

Currently, when realizing optical deflectors and optical modulators with integrated optical structures, piezoelectric materials are used as optical waveguide substrates.

Lithium niobate (hereinafter referred to as LiNbO3), which has excellent photoacoustic effects and electro-optic effects, and low light propagation loss
Lithium tantalate (hereinafter referred to as LiTaO3) crystals are widely used.

As a typical method for manufacturing a thin film optical waveguide using the crystal substrate, titanium (hereinafter referred to as Ti) metal is thermally diffused onto the surface of the crystal substrate at high temperature, and the metal is diffused onto the surface of the crystal substrate. There is a method of forming an optical waveguide layer having a refractive index slightly larger than the refractive index of the substrate. However, the thin film optical waveguide produced by this method is susceptible to optical damage and has the drawback that only very low power light can be introduced into the waveguide. Here, optical damage is defined as ``When the intensity of light input to an optical waveguide is increased by 1-, the intensity of light propagated within the optical waveguide and taken out to the outside is proportional to the intensity of the input light due to scattering. "A phenomenon where it stops increasing."
say.

As a method for manufacturing an optical waveguide that improves the optical damage,
A crystal substrate of LiNbO3 or LiTaO3 is heat treated at high temperature, and lithium oxide (hereinafter L t 20
There is a method of externally diffusing lithium (hereinafter referred to as Ll) to form a vacancy layer of lithium (hereinafter referred to as Ll) having a slightly larger refractive index than the substrate near the surface of the substrate.

According to the literature CR, the above Li,O external diffusion method increases the optical damage threshold compared to the Ti metal internal diffusion method.
, L., Holman & P., J., Cress
man.

IOC, 90, 28 April (1981)) By the way, when trying to realize an optical inverter or optical modulator using photoacoustic effects or electro-optic effects, it is necessary to increase the efficiency of each of the above effects. This becomes important in device formation. A typical example of utilizing the photoacoustic effect is a method in which a high-frequency electric field is applied to comb-shaped electrodes fabricated by photolithography on an optical waveguide to excite surface acoustic waves 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. [C, S, Tsai,
IEEE TRANSACTION SON CIRCU
ITS AND SYSTEMS, VOL, CAS
-26゜12, 1979] From the viewpoint of maximizing the above interaction, the optical waveguide layer (L
The thickness of the i vacancy layer is 1 because its refractive index change is small.
It is necessary to make it quite thick, about 0 to 100 μm, which is undesirable because the energy distribution of the guided light spreads in the thickness direction. Therefore, when a thin film optical waveguide fabricated by the above-mentioned Li2O external diffusion method is used for the optical deflector or the like, it has been difficult to realize a highly efficient device.

Furthermore, the LLO outdiffusion method involves a simple heat treatment, and therefore has the disadvantage that a channel type waveguide cannot be formed.

On the other hand, an ion exchange method is known as another method for manufacturing a thin film optical waveguide that improves optical damage. This method uses LiNb0. Or Li
By subjecting a TaO3 crystal substrate to low-temperature heat treatment, lithium ions (Li+) within the crystal substrate are
This is exchanged with ion species of thallium (T/') or silver (Ag) in the molten salt, and an optical waveguide layer having a large refractive index difference (Δn~0.12) is formed.

However, the optical damage threshold of the thin film optical waveguide fabricated by the above ion exchange method is
and A, M, Glass.

J, Appl, Phys, 52(7) Jul
y 1981:], the improvement was only about twice that of the 1゛i internal diffusion method, but it was not sufficient, and it had the problem of a long processing time of several tens of hours. .

An object of the present invention is to solve the problems of the conventional example, and to provide an externally diffusing thin film optical waveguide which has a sufficiently high optical damage threshold and has a large interaction between guided light and photoacoustic effects and electro-optic effects. The object of the present invention is to provide a method for producing the same.

Another object of the present invention is to reduce optical damage. It is an object of the present invention to provide a channel-type externally diffused thin film optical waveguide having a sufficiently high threshold and a large interaction between guided light and photoacoustic effects and electro-optic effects, and a method for manufacturing the same.

Another object of the invention is that the optical damage threshold is sufficiently high;
It is an object of the present invention to provide an externally diffusing thin film optical waveguide that requires a short processing time during production, and a method for producing the same.

In the present invention, an ion species implantation layer is formed on the surface of a lithium niobate (LiNbOs) crystal substrate or a lithium tantalate (LiNbO3) crystal substrate by implanting ion species into the crystal substrate, and the ion species implantation layer is formed by implanting ion species into the crystal substrate. Lithium (J,, D' = external diffusion thin film guiding waveguide with lattice layer formed inside the crystal substrate and lithium niobate (LiNbO3)
A crystal substrate or a lithium tantalate (LiTa03) crystal substrate is subjected to high-temperature heat treatment 17, and lithium (L) is applied to the surface of the crystal substrate.
i) The process of forming a vacancy layer and the lithium (Li
) The above object is achieved by a method of manufacturing an externally diffused thin film optical waveguide, which comprises the step of injecting ion species into the crystal substrate surface side of the vacant lattice layer to form an ion glaze implanted layer.

Hereinafter, the present invention will be explained using the drawings.

FIG. 1 is a schematic diagram showing the structure of an externally diffusing thin film optical waveguide of the present invention. In the figure, 1 is a crystal substrate, cut LiTaO
5 can be used. The Li vacancy layer is formed by subjecting the crystal substrate to high-temperature heat treatment to diffuse Li2O to the outside. Ion species implantation/#
3 can be formed by implanting ion species into the Li vacancy layer 2 using an ion exchange method or the like. In the externally diffusing thin film optical waveguide of the present invention, Li
The amount of change in refractive index of the vacant lattice layer 2 with respect to the crystal substrate J is 0.
01, and the amount of change in refractive index of the ion species implanted layer 3 is 0.1 to 0.01.
The guided light propagates through the ion species implantation layer 3. In addition, the Li below the ion species implanted Ji813
The vacancy layer 2 has the effect of capturing photoelectrons generated by light irradiation with the Li vacancy, preventing the optical waveguide from being damaged by the photoelectrons, and improving the optical (zero scratch threshold).

In order to fully utilize this effect, the Li vacancy layer 2t
Considering the depth of the guided light seeping into the crystal substrate,
It is desirable to have a thickness of 10 to 100 μm.

Next, in the present invention, a mechanism for implanting ion species into an L t NbO3 crystal substrate using an ion exchange method will be explained. The L i Nb03 crystal is a trigonal crystal as shown in FIG. 2(a), with lithium (Li) 5.
Hole 6. Niobium (Nb) 7 is arranged in order. Here, 4 is oxygen (0).

The TJiNbO3 crystal shown in Figure 2 Ca) is approximately 1000"
When heat treated at high temperature of O, as shown in Fig. 2(b), <Li
Ions are emitted to the outside and Li vacancies 9 are formed.

In the above state, for example, by ion exchange method, Ag
, T1. When an ion species such as 10 is implanted, the ion species 10 enters the position of the Li vacancy, as shown in Figure 2 (c). In a normal ion exchange mechanism, Li ions and implanted ions are exchanged at the same time, which results in a long processing time. On the other hand, in the case of the present invention, since the Li sites are diffused outside by heat treatment and are vacant before ion exchange treatment, ion species can be easily injected, and the treatment time can be shortened compared to the usual ion exchange method. An optical waveguide having an even larger refractive index difference can be realized.

Further, since the optical waveguide layer made of the ion species implantation 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, ]1 is the frequency IG)I7. 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 amplitude distribution of the guided light in the optical waveguide fabricated by conventional Illi internal diffusion. Shows energy distribution. Third
As shown in the figure, the intensity of the surface acoustic wave has a peak at a depth of about 0.5 μm from the substrate surface. On the other hand, the peak of the energy distribution 12 of the optical waveguide of the present invention also exists at a depth of about 0.7 μm from the surface, and the proportion of overlap with the energy distribution 11 of surface acoustic waves is similar to that of the conventional optical waveguide. The surface acoustic wave becomes larger dramatically, and the interaction between the guided light and the surface acoustic wave increases. In the present invention, in order to obtain the above-mentioned large interaction, the optical waveguide layer, that is, the ion species implantation layer, is preferably as thin as possible, preferably about 1 to 2 μm.

Hereinafter, the present invention will be specifically explained with reference to Examples.

[Example 1] X-cut Li Nb O, crystal substrate (2 dimensions thick in the X direction.

-plane (for example, X
1 side) was polished to a flatness within a few Newton rings, and then subjected to normal ultrasonic cleaning using methanol, acetone, and pure water, and dried by blowing nitrogen gas. Said cleaning to out-diffuse the mustard 120 on the LiNbO5 substrate.

Stand the dried substrate in a fused silica holder and hold it for 1000
'(j) was set in a thermal diffusion furnace. Dry O gas was introduced into the diffusion furnace at a flow rate of 2 l/min as an atmospheric gas. The temperature inside the furnace was increased from room temperature to 1000°C at a rate of 16° After 1 hour, after the temperature in the furnace became constant, it was held at 1 (l fJ Furthermore, the power supply to the second expansion (if furnace) was raised to 6
() Allow to cool from 0°C to room temperature.

The reason for rapidly cooling the substrate from 101)0'O to 600°0 is to prevent the formation of a LiNbO3 substrate within the substrate due to gradual cooling and to form an optical waveguide with low optical propagation loss. If a light propagation loss on the order of i dB/m can be tolerated, a single thermal diffusion furnace can be used to
It may be formed by being left to cool.

In order to measure the characteristics of the Li, 0 externally diffused optical waveguide obtained after cooling, we introduced He-Ne light with a wavelength of 6328 from a rutile prism and measured the optical propagation loss.
．． It was found that a high-quality optical waveguide of 3 dB/segment was fabricated. This was approximately the same optical propagation loss as an optical waveguide fabricated by internally diffusing Ti to a thickness of 200 mm. When the propagation mode was measured by changing the angle of incidence on the prism, T
Four modes could be observed in the E mode. In addition, when we measured the state of optical damage by changing the input light intensity and the ratio of input light to output light with two input beam diameters, we found that the optical waveguide due to internal Ti diffusion was Compared to optical damage caused by external diffusion of Li2O, the power consumption is 20 mW.
A stable output value was obtained for several hours even with input light of .

From this, it was found that by external diffusion of Li, 0, it was possible to fabricate an optical waveguide with the same optical propagation loss and a damage threshold about 20 times higher than that produced by internal Ti diffusion. . The Li2O external diffusion depth was determined from the mode index and was approximately 10 μm.
Thus, a thick Li vacancy layer was formed with a refractive index difference Δn of 0.01 with respect to the substrate.

After measuring the optical properties, the above-described cleaning of the substrate was repeated again to clean the substrate surface, and then ion exchange treatment was performed. For ion exchange treatment, special grade AgN0. A crystal piece was placed for 30y, and the washed and dried Li2O externally diffused substrate was placed on top of it with the surface on which the Li vacancy layer was formed facing up, and the beaker was placed in a thermal furnace and maintained at a temperature of 330°0 for 6 hours. . It is desirable to stir the molten salt during ion exchange. Also, at this time, AgNO
Care was taken to allow the No 2 gas generated from the S solution to be discharged to the outside of the furnace. After such ion exchange treatment, the power to the furnace was turned off and the furnace was allowed to stand until the temperature inside the furnace returned to room temperature. Thereafter, the solidified substrate and AgNOs were taken out from the heat furnace while still in a beaker, and purified water heated to about 60 to 70°C was poured into the beaker and left until the substrate and AgNO3 were separated. After 1 hour, only the separated substrate was taken out and further washed with warm water for several hours until the AgNO3 microcrystals were completely removed from the substrate surface. If the microcrystals were difficult to remove, the surface of the optical waveguide was lightly polished using an abrasive such as colloidal silica.

In order to investigate the characteristics of the planar type externally diffusing thin film optical waveguide of the present invention, which was created by him, we used a rutile prism to
The 28 He-Ne lights were again introduced into the Z direction within the waveguide plane, and the optical propagation loss was measured, and a value of 0.6 dB/c1n was obtained. Optical damage [7 threshold is 24 r with He-Ne light]
The values of r, 'W/α were obtained, and for comparison, X-cut LiN
When only ion exchange is performed under the same conditions without externally diffusing Li and O into the bO3 substrate, the optical propagation loss is 1 dB/
crn, the optical damage threshold was 41 nW/rrn.

The propagation mode of the externally diffused thin film optical waveguide produced in this example was almost the same propagation mode TEo of the optical waveguide produced only by the ion exchange method under the same conditions.

Next, after the measurement, the optical waveguide of this example was cleaned again.

After drying, use a spinner to apply positive photoresist to a thickness of 1
It was spinner coated to ~1.5 μm and closely exposed using a negative mask of comb-shaped electrodes, and developed so that only the comb-shaped electrode portions were not left. After washing with water, drying and loading into a vacuum evaporator, I
Evacuation was performed to X 10-6 Torr, and Al (thickness: 1500 mm) was deposited by EB evaporation. After the deposition, the Al film on the photoresist was removed by lift-off by immersion in acetone for several minutes, and only the comb-shaped electrode portion was formed on the optical waveguide. In this case, the comb-shaped electrode has a surface acoustic wave center wavelength of 60011117. Therefore, the electrode width and electrode spacing of the comb-shaped electrodes are both 1.
It was 45 μm. In this way, a comb-shaped electrode was provided on the optical waveguide of the present invention to produce an optical waveguide type optical deflector.

On the other hand, an optical waveguide type optical deflector was fabricated by forming comb-shaped electrodes using the same lift-off method as described above on an optical waveguide fabricated by thermally diffusing Ti to a thickness of 200 mm.
Performance was compared with an optical waveguide type optical deflector manufactured using the optical waveguide of the present invention under the following conditions. In both cases, a He-Ne laser with a wavelength of 6328 was used as the incident light, and RF power of 0.6 W was applied to the comb-shaped electrodes. When the RF frequency is 600 MIIZ, the diffraction efficiency of the optical waveguide type optical deflector fabricated on the Ti thermal diffusion optical waveguide is about three times that of the optical waveguide fabricated on the optical waveguide of this example, High diffraction efficiency was obtained. In addition, the intensity of the charged H,e-Ne laser light was measured at 1 mW or less, but the intensity was 5 mW, 10 mW, and 15 mW.
When the laser light intensity was increased by '', in the case of the Ti thermal diffusion optical waveguide, optical damage already occurred at an input intensity of 5 mW, and the output light decreased catastrophically, but in the case of this example, the input intensity increased to 15 mW. It was found that even when the output light intensity exceeded the above range, the output light intensity increased in proportion to the input light intensity, and no optical damage occurred at all.

[Example 2] Next, a second embodiment of the present invention will be described.

In the case of the first example, the ion exchange treatment was performed at 330'OAg.
The ion exchange treatment was carried out in NO, molten salt, but in the second example, the ion species was K (potassium), so the ion exchange treatment was performed in KN.
O, carried out in molten salt. The optical waveguide of the present invention was fabricated by treating an X-cut LiNbO5 substrate after external diffusion of Li and O produced under the same conditions as in the first example in a 360'Q KNOs molten salt for several tens of minutes to several hours. An optical waveguide with a low optical propagation loss of 0.4 dBAyn was fabricated. A comb-shaped electrode was fabricated on the optical waveguide using the same process as in the first example, and the frequency was 6001111z and 0.6W.
The diffraction efficiency was measured by applying an RF power of 100 to 100 nm to the comb-shaped electrodes, and almost the same results as in the first example were obtained. or,
Regarding optical damage, similar results to those of the first example were obtained. KNO, has a melting point lower than AgNOs by 360°.
AgN0. The advantage of this method is that it is possible to create a waveguide with a smaller optical propagation loss and a larger difference in refractive index than when using molten salt.

[Embodiment 3] Next, a third embodiment of the present invention, which is a channel type externally diffused thin film optical waveguide, will be described. First, Li2O external diffusion treatment was performed under the same conditions as in the first example, and then the Li2O
2O external diffusion .

After cleaning and drying the substrate again, a negative photoresist was coated with a spinner to a thickness of 1 to 1.5 μm (-, and was closely exposed using a negative mask of a channel type optical waveguide, and developed so that only the channel portion remained. After washing with water, dry
Load the vacuum evaporator and evacuate to I
500 people) were deposited. However, as a metal film, Al
The substrate is not limited to the above, and may be a metal film (eg, Ti, Au) that prevents ionic species from entering the substrate in the subsequent ion exchange treatment.

By immersing it in acetone for several minutes after vapor deposition, the A/film on the photoresist was removed by lift-off, and an Al film was formed only in the area other than the channel area. The channel width at this time was 5 μm.

Next, in order to form a layer in which ion species were implanted in the channel portion, an ion exchange treatment similar to that in the first example was performed. After the above-mentioned ion exchange treatment, the metal film was removed with an etching solution and washed again to produce a channel type optical waveguide having the structure of the present invention. If the metal film is AI, phosphoric acid may be used as the etching solution.

Similar to the first example, optical propagation loss and optical damage were measured, and almost the same results were obtained.

[Example 4] Next, a fourth embodiment of the present invention will be described.

Example 1 described above includes a wet treatment process called ion exchange treatment, but in this fourth example, the optical waveguide of the present invention is manufactured using all dry treatment processes. First, under the same conditions as the first example, LiNb
0. , Li2O external diffusion treatment is performed on the crystal substrate. Subsequently, the above-mentioned substrate is loaded into a vacuum evaporation device, evacuated to 1×10' Torr, and a metal film, for example, Ag (film thickness: 2000 mm) is formed by EB evaporation.
was deposited. It was placed in the electric furnace again and heat treated at 550°C for 1 hour. The time it takes to stand at 550°01 is 0.5 hours, and after the heat treatment, the electricity is stopped and the temperature is set at 550°.
Heat was dissipated from °C to room temperature. Through the above heat treatment, the ion species in the metal film were diffused into the substrate, and the guiding waveguide of the present invention was fabricated. After fabrication, original propagation loss and optical damage were measured in the same manner as in the WJ1 example, and almost the same results as in the first example were obtained. In this embodiment, the metal to be internally diffused is not limited to Ag, and other metals (for example, Ti, etc.) may be used.

In the above-mentioned embodiment, an example was shown in which Li NbO3 was used as the crystal substrate, but the present invention can be carried out in exactly the same way even if a Li Ta(J) crystal substrate is used.

As explained above, the present invention has the following advantages in the conventional thin film optical waveguide and its manufacturing method: 1) Increases the threshold of optical damage 2) When used in an optical deflector etc. 3) It has the effect of reducing the processing time required for manufacturing, etc. by improving the interaction with the electro-optical effect.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the structure of the externally diffused thin film optical waveguide of the present invention, and FIGS. 2(a), (b), and (c) are, respectively,
FIG. 3 is a schematic diagram illustrating the state of ion species implantation in the manufacturing process of the present invention, and FIG. 3 is a 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. DESCRIPTION OF SYMBOLS 1... Crystal substrate, 2... Li vacancy layer, 3... Ion species injection layer, 4... Oxygen, 5... Lithium, 6...
...Vacancy, 7...Niobium, 8...C axis, 9...L
i vacancy, 10... ionic species. Applicant Canon Co., Ltd. 16-

Claims (5)

[Claims] (1) An ion species implantation layer is formed on the surface of a niobium lithium oxide (LiNb0.) crystal substrate or a tantalum iff IJ lithium (LiTa03) crystal substrate by implanting ion species into the crystal substrate, and the ion species are This is an external diffusion thin film optical waveguide in which a lithium (Li) vacancy layer is formed in the crystal substrate 111 from the injection layer. (2) The lithium-ion (Li) vacancy layer is 10 to 1
The externally diffusing thin film optical waveguide according to claim 1, having a thickness of 00 μm. (3) The externally diffusing thin film optical waveguide according to claim 1, wherein the ion (11) focusing layer has a thickness of 1 to 2 μm. (4) The externally diffusing thin film optical waveguide according to claim 1, wherein the cut plane of the crystal substrate is the X plane or the two planes. (5) A process of performing high-temperature heat treatment on a lithium niobate (LiNbOs) crystal substrate or a lithium tantalate (LiTa0.) crystal substrate to form a lithium (Li) vacancy layer on the surface of the crystal substrate; A method for manufacturing an externally diffused thin film optical waveguide, which comprises the step of injecting ion species into the surface side of a crystal substrate of a child layer to form an ion species implantation layer.