JP3128332B2 - Method of forming diffused optical waveguide with improved surface morphology - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a diffusion type optical waveguide having an improved surface morphology. More specifically, the present invention relates to a dielectric (including ferroelectric) substrate such as Li
The present invention relates to a method for forming an optical waveguide having a low surface roughness on an NbO ₃ single crystal substrate.

[0002]

2. Description of the Related Art In general, a substrate made of a dielectric crystal, particularly a ferroelectric crystal having high electro-optical characteristics, for example, a single crystal of LiNbO ₃ has an effect of increasing the refractive index of the dielectric crystal. A substance, for example, Ti for LiNbO ₃ single crystal is adhered to a predetermined pattern, and this is thermally diffused into a portion near the surface of the derivative substrate, thereby locally changing the refractive index of the pattern portion of the derivative substrate. It is known that optical waveguides can be formed at elevated levels.

[0003] Such a technique is described in, for example, "Optical Integrated Circuit", co-authored by Nishihara, Haruna, and Suhara (Ohmsha (1985)).
A Ti-diffused LiNbO ₃ waveguide is illustrated and described. A waveguide formed by inwardly diffusing Ti in the surface portion of a LiNbO ₃ single crystal has an excellent electrochemical effect and an optical non-linear effect, and therefore has a high application value to an optoelectronic device. Many studies have been reported on this technology (eg, RVSchmidt &
IP Kaminiw, Appl. Phys. Lett. 25 (1974) 458).

In order to indiffuse a substance for increasing the refractive index, for example, Ti, into a ferroelectric substrate, for example, a LiNbO ₃ single crystal substrate, Ti is deposited and deposited on a predetermined pattern region on the substrate surface. Ti is treated at a temperature lower than the Curie temperature of 1080 ° C. of LiNbO ₃ but as high as possible to obtain a higher diffusion rate, for example 98 ° C.
A method of heating at a high temperature of 0 ° C. and thermally diffusing the same into a substrate is adopted (for example, Rj Holmes & DMSmyt)
h, J. Appl. Phys. 55 (1984) 3531). However, in the conventional method, there is a problem that the surface roughness of the obtained optical waveguide is increased due to a number of irregularities in the surface of the obtained optical waveguide in the thermal diffusion treatment of Ti.

When the roughness of the optical waveguide surface increases, naturally,
There is a serious disadvantage that the light propagation loss of the optical waveguide is large. For this reason, many attempts have been made to reduce the roughness of the optical waveguide surface.

[0006] As one of attempts to reduce the roughness of the surface of an optical waveguide and improve the surface morphology, a method of introducing water vapor into a heated atmosphere in a thermal diffusion treatment has been developed, and this method has already been put to practical use. ing. This technique, for example,
M.DeSario, MNArmenice, C.Canali, A.Carnera, P.Ma
zzodldi & G. Celotti, J. Appl. Phys., 57 (1985) 1482.

However, in the above technique, the surface morphology of the Ti-diffused optical waveguide is sensitively dependent on the water vapor content of the heating atmosphere, the composition of the heating atmosphere including the carrier gas, and the like. Nozawa, Sudo, Miyazawa, "Applied Physics", 59 , (1990) 99
6). Therefore, development of a method for forming a diffusion type optical waveguide having a low surface roughness on a dielectric substrate or a ferroelectric substrate has been strongly desired.

[0008]

SUMMARY OF THE INVENTION The present invention provides a method for forming an optical waveguide on a dielectric substrate by a thermal diffusion method, which has a low surface roughness and therefore a small optical propagation loss. What you want to do.

[0009]

Means for Solving the Problems In forming an optical waveguide on the surface of a dielectric substrate by a thermal diffusion method, the present inventors
The inventors have found that the form of the surface of the obtained optical waveguide can be remarkably improved by performing the ion bombardment treatment on the surface of the dielectric substrate in advance, and have completed the method of the present invention.

According to a method of forming a diffusion type optical waveguide having an improved surface morphology according to the present invention, a substance having an effect of increasing the refractive index of the dielectric is deposited and adhered on the surface of a dielectric substrate.
When forming an optical waveguide by thermally diffusing this into a portion near the surface of the substrate, prior to the attaching step, performing ion bombardment treatment on the entire surface of the substrate on which the optical waveguide is formed, The ion bombardment treatment has the effect of causing the ions to bombard the substrate surface to cause defects and / or alteration on this surface portion, and these surface defects and / or alterations are caused by the ion bombardment treatment. The recovery is performed by a thermal diffusion treatment of the deposited substance which is performed after the step.

[0011]

In the present invention, the term "dielectric" includes ordinary dielectrics and ferroelectrics. The dielectric substrate used in the method of the present invention is one conventionally used, for example, LiNbO ₃
The substrate can be selected from a single crystal, a LiTaO ₃ single crystal, a KNbO ₃ single crystal, and the like.

In the method of the present invention, the thermal diffusion material used to increase the refractive index of the dielectric substrate is selected from those conventionally used in the art, for example, transition metal elements such as Ti and rare earth elements such as Er. You can choose. The rare-earth element causes a light-emitting effect in the waveguide by diffusing the rare-earth element simultaneously into a diffusion-type waveguide made of a transition metal element.

In the method of the present invention, first, an ion bombardment treatment is performed on the entire surface of the surface of the dielectric substrate including at least the region where the optical waveguide of the predetermined pattern is to be formed. This ion bombardment treatment involves at least a noble gas (ie, He, Ne, Ar, Kr) in order to impart ions of a substance that does not adversely affect the formation of the optical waveguide and its performance, preferably high efficiency. , Xe and Rn) in a gas atmosphere containing ions. There is no particular limitation on the type of the ion bombardment treatment as long as the ions bombard the surface of the dielectric substrate and can give a desired effect thereto.
An ion beam sputtering method, an ECR sputtering method, or the like is used.

In the method of the present invention, when the entire surface of the surface of the dielectric substrate on which the optical waveguide of the predetermined pattern is formed is subjected to the ion bombardment treatment, at least a part of the constituent atoms of the surface portion is sputtered and emitted. Point defects such as ion implantation, crystal defects, lattice defects, etc.
And / or alteration occurs. Here, the alteration includes, for example, amorphization of a portion near the surface of a crystal substrate, and a change in a polarization structure due to a charge impact in a ferroelectric crystal. Although the surface portion of the dielectric substrate is essentially an insulator, the surface portion subjected to the ion bombardment treatment is turned into a conductor, and as described above, the crystal defects that become conductive carriers near the surface of the derivative substrate by the ion bombardment treatment It is suggested that the occurrence has occurred.

Next, a surface of the dielectric substrate which has been subjected to the ion bombardment treatment and modified, has a refractive index improving substance, for example, Ti
Are deposited according to a predetermined waveguide pattern. Although there is no particular limitation on the method of depositing and depositing the refractive index improving substance, generally, a vacuum deposition method such as an evaporation method and a sputtering method is used, and a predetermined amount of the refractive index improving substance is ionized on the dielectric substrate. Deposited on impact treated surface.

Next, the material for improving the refractive index on the dielectric substrate is subjected to a thermal diffusion treatment. The conditions of the heat diffusion treatment are appropriately set according to the type of the refractive index improving substance and the dielectric substrate to be used. In the case of the LiNbO ₃ substrate, the conditions are generally 980 to 1 in a moist oxygen gas stream atmosphere.
It is performed at a temperature of 050 ° C.

In the method of the present invention, as described above, modification such as crystal defects and alteration occurs over the entire surface of the dielectric substrate on which the optical waveguide is formed by the ion bombardment treatment. By the above-mentioned thermal diffusion treatment, the refractive index improving substance is thermally diffused into the dielectric substrate via the surface modification layer, and at the same time, the defects and alteration on the dielectric substrate surface are restored to a normal state. An optical waveguide having a good surface morphology, for example, having a relatively smooth surface is formed.

In the thermal diffusion in the method of the present invention, when the heating temperature is extremely low, for example, a temperature (Ishizaka) which is half the melting point (absolute temperature), which is known as the optimum epitaxial growth temperature of the crystal thin film layer. , Journal of the Physical Society of Japan, 44 (198
9) If lower than 408), the restoration of defects and alterations on the dielectric substrate surface may be insufficient. Further, if the heating temperature is higher than the phase transition temperature such as the Curie point, the obtained waveguide has drawbacks such as not exhibiting the original physical properties (such as the electro-optical effect).

In the method of the present invention, the reason why the ion bombardment treatment applied to the surface of the dielectric substrate improves the surface morphology of the optical waveguide formed thereon has not yet been sufficiently clarified. Crystal defects, oxygen vacancies, and alterations formed on the surface of the dielectric substrate due to the impact promote the rate of thermal diffusion and diffusion reaction of the refractive index improving substance,
It is considered to prevent precipitation of unreacted substances, which is one of the factors affecting the roughness of the optical waveguide surface.

[0020]

The present invention is further described by the following examples.

Example 1 LiNbO ₃ substrate (Z-cut, manufactured by Yamatosu Ceramics Co., Ltd., 3 inch substrate, 0.5 mm thick surface was wet-cleaned with acetone, ethyl alcohol, a surfactant and ultrapure water. The substrate was set on an electrode of an RF-sputtering apparatus, and argon was applied to the entire surface of the substrate in a pure argon atmosphere at a pressure of 0.3 Pa and an RF output of 100 W for 15 minutes. An ion bombardment treatment was applied.

By the above argon ion bombardment treatment, the surface to be treated of the dielectric substrate turns black, and its resistance value is 200 to 3
It dropped to 00Ω.

An optical waveguide pattern of Ti was drawn on the processed surface of the substrate by a vacuum deposition method and a lift-off method using a photolithographic method. At this time, the evaporation time was adjusted so that the thickness of the Ti evaporation layer was 800 nm.

The above-described dielectric substrate on which Ti was deposited was placed in an electric furnace and subjected to a thermal diffusion treatment at 980 ° C. for 20 hours. At this time, the synthetic air was introduced into the electric furnace while bubbling the ultrapure water at room temperature.

The surface morphology of the optical waveguide obtained by the above operation was photographed with an optical microscope. The photograph is shown in FIG.
As is evident from FIG. 1, formed on the dielectric substrate,
The surface of the optical waveguide having a width of 7 μm (the stripe portion extending in the lateral direction at the center of the photograph) had relatively low roughness. Also, the blackening of the surface of the obtained dielectric substrate,
It was eliminated by heat diffusion treatment, and the surface conductivity was restored to an insulator. In addition, a Mach-Zehnder type intensity modulator was manufactured to show that the obtained optical waveguide exhibited light modulation characteristics due to the electro-optic effect, similarly to an optical waveguide manufactured without performing ion bombardment treatment by a conventional method. confirmed.

Comparative Example 1 An optical waveguide (7 μm in width) was formed on a LiNbO ₃ substrate in the same manner as in Example 1. However, the argon ion bombardment treatment was omitted. FIG. 2 shows the surface of the obtained optical waveguide.
As is clear from FIG. 2, the surface roughness of the optical waveguide was clearly lower than that of Example 1.

[0027]

According to the method of the present invention, it is possible to form an optical waveguide having an improved surface morphology and a relatively low roughness on a dielectric substrate. In addition, in the method of the present invention, crystal defects, alteration, etc. occur in the vicinity of the substrate surface due to the ion bombardment treatment, causing blackening, conductivity, etc., which are restored and eliminated by the subsequent thermal diffusion treatment. Thus, an optical waveguide having good performance is formed. The optical waveguide obtained by the method of the present invention has high reliability in both optical waveguide characteristics such as light propagation loss and process reproduction / management, and has extremely high value as an optoelectronic device process technology. .

[Brief description of the drawings]

FIG. 1 is an optical microscope photograph showing a surface morphology of an optical waveguide obtained by a method (Example) of the present invention.

FIG. 2 is an optical microscope photograph showing a surface morphology of an optical waveguide obtained by a conventional method (comparative example). 1 and 2
In the figure, the stripe-shaped portion extending in the lateral direction at the center portion is the optical waveguide.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-59-157602 (JP, A) JP-A-4-251804 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 6/12-6/14 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. A method of depositing and adhering a substance having an effect of increasing the refractive index of the dielectric on the surface of a dielectric substrate, and thermally diffusing the substance into a portion near the surface of the substrate to form an optical waveguide. Prior to the adhering step, an ion bombardment process is performed on the entire surface of the substrate on which the optical waveguide is formed, and the ion bombardment process causes the ions to collide with the surface of the substrate. It has an effect of causing defects and / or alteration, and these surface defects and / or alteration are recovered by the thermal diffusion treatment of the deposited substance after the ion bombardment treatment. A method for forming a diffusion type optical waveguide having an improved surface morphology. 2. The method according to claim 1, wherein the ion bombardment treatment is performed in a gas atmosphere containing at least rare gas ions. 3. The method according to claim 1, wherein the dielectric substrate is a LiNbO ₃ single crystal substrate, and the refractive index improving substance is Ti.
The method described in.