JPH0437701A - Formation of optical waveguide - Google Patents

Abstract

PURPOSE:To well confine light to decrease the transmission loss of light by selectively implanting a high-refractive index material having the refractive index higher than the refractive index of the substrate into the part of the substrate to be formed with the optical waveguide, thereby forming the ion implanted region so as to steepen the refractive index distribution near the boundary between an optical waveguide and a substrate. CONSTITUTION:The high-refractive index material (titanium ion) 22 having the refractive index higher than the refractive index of the LN substrate 21 is selectively implanted to the part of the LN substrate 21 to be formed with the optical waveguide to form the ion implanted region 23 and, therefore, the ion implanted region 23 is formed by the effect of the ion implantation having high kinetic energy. A diffusing material layer 24 consisting of titanium is formed on this ion implanted region 23 and thereafter the LN substrate 21 is subjected to a heating treatment, by which the diffusing material layer 24 is diffused into the LN substrate 21 to form a high- refractive index layer 25 and, therefore, the good high-refractive index layer 25 is formed in a short period of time. The strains of the crystal structure by the implanted atoms are annihilated by the heating treatment, by which the good crystal structure is obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention is applicable to optical switches, optical modulators, optical integrated circuits (first
The present invention relates to a method of forming an optical waveguide having a steep high refractive index distribution and excellent optical confinement effect in an optical device such as C) or a front component.

[Conventional technology] In recent years, with the development of thin film production technology and microfabrication technology, optical switches, front filters, optical modulators, and optical integrated circuits (optical ICs) have become increasingly popular.
As integration progresses in optical devices and optical components such as L), it has become essential to form an optical waveguide on a semiconductor substrate (L), which is a dielectric substrate.

This substrate-type optical waveguide is indispensable for achieving higher performance, higher reliability, and lower cost in optical devices and optical combo networks.For example, in optical components, it is necessary to When efficiently joining optical devices and optical fibers to optical waveguides, it maintains optical axis adjustment during connection and stable alignment after connection, making it highly resistant to environments such as vibration and temperature. ing.

As a substrate material for this substrate-type optical waveguide, for example, in optical switches and hybrid ICs, two-year-old glass is used because of its low optical transmission loss and high electro-optic properties (electro-optic effect). Lithium oxide (L iN
Dielectric materials such as bo3) are often used, and in optical devices such as semiconductor lasers, gallium arsenide (GaAs) and innoum phosphorus are also suitable materials for high-speed electronic circuits and high-speed photodetectors. Compound semiconductor materials such as (InP) are often used.

Here, with reference to FIG. 4, a lithium niobate (LiNbos) substrate (hereinafter simply LN
A method for forming an optical waveguide on a substrate (referred to as a substrate) will be described.

2 [See FIG. 4(a)] Photoresist 2 is applied over the entire surface 1a of the LN substrate 1 so that the thickness is uniform.

■ [See FIG. 4(b)] A mask 3 with an optical waveguide pattern drawn thereon is placed on a predetermined position on the photoresist 2, and the mask 3 is exposed to ultraviolet light 4.
The pattern of the optical waveguide is transferred onto the photoresist 2.

2 [See FIG. 4(c)] By developing the photoresist 2, the photoresist 2a in the portion where the optical waveguide is to be formed is removed from the LN substrate l, and a window 5 is formed in the photoresist 2.

■ [See FIG. 4(d)] A diffusion material layer 6 is uniformly formed on the photoresist 2 and the window 5 on the LN substrate l by sputtering, vacuum evaporation, EB evaporation, ECR sputtering, etc. .

This diffusion material layer 6 is made of a material having a higher refractive index than the LN substrate 1, such as titanium (Ti) or tantalum (Ta).
It is composed of etc.

(2) [See FIG. 4(e)] Photorenost 2 is completely removed from the LN substrate 1 using an organic solvent or the like. As a result, the diffusion material layer 6 remains only on the window 5 portion on the LN substrate l, that is, on the portion where the optical waveguide is to be formed.

■ [See Figure 4 (J)] This LNN substrate is inserted into a diffusion furnace and heat treated at a predetermined temperature to diffuse the diffusion material layer 6 into the L'N substrate 1, which is higher than the LN substrate 1. A high refractive index layer 7 having a refractive index is formed. This high refractive index layer 7 becomes an optical waveguide 8.

Among the above steps, steps ① to ① are a pattern forming method called the so-called lift-off method.

FIGS. 5 and 6 are diagrams showing an example of the structure of an optical waveguide formed on an LN substrate by the above method.

On the surface 1a of the LN substrate 1, an optical waveguide 8 is formed in a straight line.

As shown in FIG. 6(a), the optical waveguide 8 is made of a high refractive index material (titanium (T
i)) is thermally diffused to form a high refractive index layer 9.

Here, FIG. 6(a) shows SIMS (Seeondary
Ion Mass 5petroIIletry:
High refractive index material (titanium (T)
This is reference data obtained by measuring the concentration distribution of i)).

In this high refractive index layer 9, a high refractive index material (titanium (
T i)) The atoms are distributed such that their concentration gradually decreases from the upper surface Ia of the LNN substrate toward the inside.

The horizontal direction (x direction) and the depth direction (
The refractive index (n) distribution in the d direction) is shown in Figures 6(b) and (c).
), it depends on the concentration distribution of high refractive index material (titanium (Ti)) atoms and shows a relatively gentle curve).

The method for forming the optical waveguide 8 has been described in detail above, and the optical waveguide 8 includes a diffusion material layer containing atoms having a low refractive index on the LNN substrate, such as a diffusion material layer such as magnonium oxide (MgO). It can also be formed by forming the diffusion material layer on a portion of the L-section substrate 1 where the optical waveguide is not formed, and diffusing this diffusion material layer onto the portion of the LNN substrate where the optical waveguide is not formed, thereby reducing the refractive index on the LNN substrate other than the optical waveguide. can do.

[Problems to be Solved by the Invention] By the way, in the method for forming the optical waveguide 8 described above, the diffusion material layer 6 is diffused into the LN substrate 1 by thermal diffusion, so that the direction of diffusion is directed toward the surface of the LN substrate 1. It becomes isotropic as if drawing a semicircle toward the inside (isotropic diffusion), or it becomes very close to it.

Therefore, when trying to obtain an optical waveguide with a predetermined thickness, the width of this optical waveguide also increases, making it extremely difficult to control the thickness and width of the optical waveguide at the same time. There was a problem that it was difficult to obtain a wave path.

Furthermore, since the optical waveguide 8 is formed by simple diffusion, the concentration gradient of the diffused atoms cannot be made steep near the boundary between the optical waveguide 8 and the LNN substrate color, but must be gradual. I don't get it. Therefore, the refractive index distribution becomes gentle, making it extremely difficult to obtain a good optical waveguide, resulting in a problem that the optical confinement effect is weakened.

When the optical confinement effect weakens, transmission loss especially in curved optical waveguides of optical devices increases, which poses a major problem in optical transmission.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a method for forming a low-loss optical waveguide that has a steep refractive index distribution and can satisfactorily confine light.

[Means for Solving the Problems] In order to solve the above problems, the present invention employs the following method of forming an optical waveguide. That is, a method for forming an optical waveguide in a dielectric substrate, in which a high refractive index material having a refractive index higher than that of the substrate is selectively implanted into a portion of the substrate where the optical waveguide is to be formed, thereby forming an ion implantation region. , a diffusion material layer made of a high refractive index material having a higher refractive index than the substrate is formed on the ion implantation region, and then the diffusion material layer is diffused into the substrate by heat-treating the substrate. It is characterized by having a high refractive index layer.

[Function] In the present invention, the high refractive index layer formed in the portion of the dielectric substrate where the optical waveguide is to be formed makes the refractive index distribution in the vicinity of the boundary between the optical waveguide and the substrate steep. Provides good confinement and reduces optical transmission loss.

[Embodiment] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

First, a method for forming an optical waveguide according to the present invention will be explained based on FIG. 1, taking an L substrate (dielectric substrate) as an example.

■: See FIG. 1(a): First, prepare the LN substrate 21 with a clean surface. Next, by focusing and scanning high refractive index material (titanium (T1) ions) 22 accelerated at an acceleration voltage of 25 kV using an ion implantation device in accordance with the optical waveguide pattern, an ion dose of 5 is applied to the upper surface 21a of the LN substrate 2I. x l O”/
An ion implantation region 23 of cm' is formed. For example, in the case of a 2×2 substrate type optical switch, the shape of the ion implantation region 23 is approximately 6 μm wide in the optical coupling region and 4 μm apart between the optical waveguides.

(2) [See FIG. 1(b)] A diffusion material layer 24 is formed over the entire area above the ion implantation region 23 by a lift-off method. This diffusion material layer 24 is LN
N substrate: 200mm thick titanium (
T i)I film. This thickness is the same as that of an optical waveguide made by the conventional thermal diffusion method.

■: Refer to Figure 1(c)-1 Insert the LN substrate 21 into a diffusion furnace, and the maximum temperature is 1050°C.
Heat treatment is performed for 4 hours to diffuse the diffusion material layer 24 into the substrate 21 to form a high refractive index layer 25. This high refractive index layer 25 becomes an optical waveguide 26.

As explained above, according to the optical waveguide forming method of the above embodiment, in the portion of the LN substrate 2I where the optical waveguide is to be formed,
A high refractive index material (titanium) with a higher refractive index than the LN substrate 21
Since the ion implantation region 23 is formed by selectively implanting the Ti) ions 22, the ion implantation region 23 can be formed in a short time due to the effect of ion implantation with high kinetic energy.

Further, a diffusion material layer 24 made of titanium (Ti) is formed on this ion implantation region 23, and then the LN substrate 21
By heating the diffusion material layer 24, the LN substrate 2
1 to form the high refractive index layer 25, ion diffusion is promoted depending on the amount of ion implantation due to partial destruction of the crystal structure due to ion implantation and distortion of the crystal structure due to implanted atoms between crystal lattices. As a result (diffusion promotion effect), this new ion implantation region becomes an ion diffusion source, so the diffusion rate increases, and a good high refractive index layer 25 can be formed in a short time.

In addition, since the crystal structure is rearranged by heat treatment, partial destruction of the crystal structure due to ion implantation and distortion of the crystal structure due to implanted atoms between crystal lattices disappears by heat treatment, resulting in a good crystal structure. can.

As described above, according to the above embodiment, a method for forming a low-loss optical waveguide that has a steep refractive index distribution and can effectively confine light by utilizing the diffusion promoting effect of ion implantation. can be provided.

The method for forming the optical waveguide 26 has been described above in detail.
The optical waveguide 26 can also be formed by forming the diffusion material layer 24 over the entire upper surface 21a of the optical waveguide 1, then performing heat treatment, and removing the diffusion material layer 24 on the LN substrate 21 other than the optical waveguide 26 by polishing or the like. I can do it.

Further, as a means for forming the ion implantation region 23, L
A high refractive index material (titanium (T) with a higher refractive index than the N substrate 21
i) Ions) 22 are selectively implanted, but the invention is not particularly limited to the above embodiments, as long as it can form an ion implantation region similar to the ion implantation described above.

FIGS. 2 and 3 are diagrams of a 2×2 substrate type optical switch (hereinafter abbreviated as an optical switch) 31 showing an example of the structure of an optical waveguide formed by the above method.

This optical switch 31 includes two optical waveguides 3 having an optical coupling region 33 on the upper surface 32a of an LN substrate (dielectric substrate) 32.
4.35 was formed.

An electrode 36 is formed on the optical waveguide 34 of the optical coupling region 33, and electrodes 37 and 38 are formed on the optical waveguide 35.

As shown in FIG. 3, the optical waveguide 34 (35) is made by implanting a high refractive index material (titanium (Ti) ions) into the portion of the LN substrate 32 where the optical waveguide is to be formed by ion implantation. The ratio layer is 40. Here, Fig. 3(a)
is SIM S (S secondary Ion M
This is reference data obtained by measuring the concentration distribution of a high refractive index material (titanium (Ti)) in the cross-sectional direction by ass spectrometry (secondary ion mass spectrometry) or Auger spectroscopy.

In this high refractive index layer 40 , the high refractive index material (titanium (Ti) atoms) extends inward from the surface 32 a of the LN substrate 32 into the boundary region 4 between the high refractive index layer 40 and the LN substrate 32 .
The distribution is such that the concentration decreases steeply at 1. The refractive index (n) distribution in the horizontal direction (X direction) and depth direction (d direction) of this high refractive index layer 40 is as shown in FIGS. 3(b) and (c). It depends on the concentration distribution of (Ti) atoms), and it is clearly divided into a high refractive index area near the center of the high refractive index layer 40, a sharp change in refractive index in the boundary region 41, and a low refractive index in the LN substrate 32. has been done.

The electrode 36 is composed of two layers: an upper electrode layer 36a and a lower electrode buffer layer 36b.
(38) also has exactly the same configuration.

In the optical switch 31 configured as described above, light incident from one end 34a of the optical waveguide 34 is emitted outward from the other end 34b when no voltage is applied between these electrodes, or When a voltage is applied between the poles of these 1, the light is transferred from the optical waveguide 34 to the optical waveguide 35 in the optical coupling region 33, and is emitted outward from one end 35a of the optical waveguide 35. That is, it operates as an optical switch.

As explained above in detail, this light switch 3! Also, the high refractive index material (titanium (T1) atoms) decreases steeply from the surface 32a of the LN substrate 32 toward the boundary region 41 between the high refractive index layer 40 and the LN substrate 32.
Since the concentration is distributed, the optical switch 3 has a steep and good refractive index distribution, and has a low-loss optical waveguide 34 and 35 that can effectively confine light.
1 can be provided.

Effects of the Invention] As described in detail above, according to the present invention, there is provided a method for forming an optical waveguide on a dielectric substrate, in which a portion of the substrate where the optical waveguide is to be formed has a refractive index lower than that of the substrate. Since the ion implantation region is formed by selectively implanting a material with a high refractive index, the ion implantation region can be formed in a short time by implanting ions having high kinetic energy.

Further, a diffusion material layer made of a high refractive index material having a higher refractive index than the substrate is formed on the ion implantation region, and then the diffusion material layer is diffused into the substrate by heat-treating the substrate. As a high refractive index layer is created by ion implantation, ion diffusion is promoted depending on the amount of ion implantation due to partial destruction of the crystal structure due to ion implantation and distortion of the crystal structure caused by implanted atoms between crystal lattices. promotion effect)
Since this new ion implantation region further serves as an ion diffusion source, the diffusion rate increases, and a good high refractive index layer can be formed in a short time.

In addition, since the crystal structure is rearranged by heat treatment, partial destruction of the crystal structure due to ion implantation and distortion of the crystal structure due to implanted atoms between crystal lattices are eliminated by this heat treatment, and a good crystal structure is regenerated. be done.

As described above, by utilizing the diffusion promoting effect of ion implantation, it is possible to provide a method for forming a low-loss optical waveguide having a steep refractive index distribution and good optical confinement.

[Brief explanation of the drawing]

1 to 3 are diagrams showing one embodiment of the present invention, and FIG. 1 is a process explanatory diagram showing a method for forming an optical waveguide;
2 and 3 are diagrams of a substrate type optical switch showing an example of the structure of an optical waveguide formed by the above method, and FIGS. 4 to 6 are diagrams showing a conventional method for forming an optical waveguide. FIG. 4 is a process explanatory diagram showing a method for forming an optical waveguide, and FIGS. 5 and 6 are diagrams showing an example of the structure of an optical waveguide formed by the above method. I LN substrate (dielectric substrate), high refractive index material (titanium (Ti) ion), ion implantation region, diffusion material layer, high refractive index layer, optical waveguide, Figure 1 h), 2 × 2 substrate type optical Switch (optical switch LN substrate (dielectric substrate), optical coupling region, optical waveguide, 38... electrode, high refractive index layer.

Claims (1)

[Claims] A method for forming an optical waveguide in a dielectric substrate, the method comprising: selectively implanting a high refractive index material having a higher refractive index than that of the substrate into a portion of the substrate where the optical waveguide is to be formed, forming an ion implantation region; A diffusion material layer made of a high refractive index material having a higher refractive index than the substrate is formed on the ion implantation region, and then the substrate is heat-treated to diffuse the diffusion material layer into the substrate to achieve a high refractive index. 1. A method for forming an optical waveguide, the method comprising forming an optical waveguide as a layer.