JPH06174908A - Method of manufacturing waveguide diffraction grating - Google Patents

Abstract

(57) [Summary] [Object] To provide a method for manufacturing a ferroelectric optical waveguide type diffraction grating with a small number of manufacturing steps. [Structure] On a ferroelectric substrate such as MgO-doped LiNbO ₃ which is polarization-inverted in a desired diffraction grating pattern in advance, a liquid phase epitaxial growth method (LPE method) is used to form a strong surface such as LiNbO ₃ having an uneven surface. A dielectric optical waveguide type diffraction grating is grown. After that, the polarization direction is made uniform by applying an electric field (poling).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical integrated device used as a light source for optical disk devices, laser printers and other optical application devices.

[0002]

2. Description of the Related Art Hiroshi Nishihara et al., "Optical Integrated Circuits" (1
985), pp 226-228, as shown in FIG. 2A, a refractive index modulation formed by periodically exchanging protons on an optical waveguide formed by Ti diffusion on a LiNbO ₃ substrate. A type diffraction grating is described.

In addition, in Japanese Patent Laid-Open No. 3-8134, FIG.
As shown in (b), Corning 7059 glass is formed as a buffer layer on a LiNbO ₃ substrate, and further,
An example of a relief type diffraction grating in which a diffraction grating layer TiO ₂ is formed on a buffer layer and then finely processed by lithography using an electron beam and ion etching is described.

[0004]

However, with the method shown in FIG. 2A, it is difficult to fabricate a diffraction grating having a short period equal to or less than that of the waveguide layer due to lateral diffusion of protons ( Nishihara Hiro et al., "Optical Integrated Circuits" (1985
Year), p. 228). Generally, in the refractive index modulation type diffraction grating, the coupling between the guided mode and the radiation mode is weak,
It is difficult to fabricate a device such as a condensing grating (diffraction grating) / coupler that converts a waveguide mode and a radiation mode (Hiro Nishihara et al., "Optical Integrated Circuit" (1985).
Year), p. 215).

On the other hand, the method shown in FIG. 2 (b) is complicated in many manufacturing steps such as forming a SiO ₂ buffer layer in order to prevent the peeling and cracking of the grating layer.

The object of the present invention is to obtain the LiN shown in FIG.
It is an object of the present invention to provide a method of manufacturing a fine diffraction grating which couples a waveguide mode and a radiation mode with high efficiency and has a small number of manufacturing steps, in order to improve a relief type diffraction grating on a bO ₃ optical waveguide.

[0007]

In order to solve the above-mentioned problems, the present invention provides a ferroelectric optical waveguide type diffraction grating having irregularities on the surface thereof, which has been polarization-inverted into a desired diffraction grating pattern in advance. An uneven portion of the diffraction grating is formed by a liquid phase epitaxial growth method (hereinafter, LPE method) using a dielectric substrate. Therefore, LiNbO ₃ is used as the ferroelectric optical waveguide type diffraction grating and grown by the LPE method. Further, the flux used in the LPE method is vanadium pentoxide (V ₂ O ₅ ) or boron trioxide (B ₂ O ₅ ).
₂ O ₃ ), or lithium fluoride (LiF), potassium fluoride (KF), or boron trioxide (B ₂ O ₅ ) and molybdenum trioxide (MoO ₃ ), or boron trioxide (B ₂ O ₃ ). And tungsten trioxide (WO ₃ ).

[0008]

The substrate is made of, for example, Mg-doped LiNb whose surface is periodically poled by diffusion of Ti in advance.
O ₃ is used.

On the above substrate, for example, V ₂ O ₅ is used for L
When a LiNbO ₃ thin film is grown by the PE method, a film maintaining its polarization direction grows on the substrate. At this time, since the film growth rates in both polarization directions are different, LiNbO
₃ Periodic unevenness is formed on the surface of the thin film (Presentation of the 39th Japan Federation of Applied Physics Lectures (1992,
Spring), Vol. 1, p. 241). That is, a waveguide type diffraction grating is formed.

Further, if an electric field is applied to the LPE, the film growth rate in both polarization directions can be controlled, so that the unevenness of a desired step can be obtained (Soviet Physics Solid State magazine, Vol. 29). , 9 (September, 1987), pp. 1578-1580).

After that, when an electric field is applied (poling) in parallel to the polarization direction, the polarization is aligned in the electric field direction (Integrated Photonics Research (Integrated Photonics Research).
Photonics Research 1992), pp144-1
45). As a result, a waveguide type diffraction grating with a uniform polarization direction can be obtained.

[0012]

1 is a sectional view of a ferroelectric diffraction grating of the present invention,
FIG. 3 is a manufacturing process drawing thereof. The ferroelectric diffraction grating of the present invention can be manufactured by combining materials.

For example, LiNbO ₃ doped with MgO is used as the substrate, and LiNbO ₃ is used as the diffraction grating.

FIG. 3 is a process diagram of a method of manufacturing the above ferroelectric diffraction grating by liquid phase epitaxial growth and poling.

First, a 5 mol% MgO-doped LiNbO ₃ substrate 51 whose + c surface is optically polished as shown in FIG.
Ultrasonic cleaning in acetone and pure water, and dry quickly.

Then, as shown in FIG. 3B, a 30 Å Ti film 81 is formed on the + c surface by sputtering.

Then, as shown in FIG. 3C, a Ti film 81 is formed.
A photoresist 82 is applied on the top by a spinner, and the photoresist 82 is patterned by using a photoresist having a concave portion of the diffraction grating opened therein (FIG. 3D).

Then, using the photoresist as a mask, CF
_The Ti film 81 is patterned by RIE using ₃ Cl gas, and the photoresist 82 is removed (FIG. 3E).

Next, the substrate is placed in a diffusion furnace and heat-treated at about 1100 ° C. for about 10 minutes in an Ar atmosphere containing water vapor through a hot water bubbler at about 80 ° C. At the time of cooling, the atmosphere was changed to O ₂ containing water vapor. As a result, as shown in FIG.
The domain inversion part 56 is formed on the surface.

Then, a LiNbO ₃ single crystal thin film 52 is formed on the + c surface of the substrate 51 by the liquid phase epitaxial crystal growth method.

The melt at the time of epitaxial growth is 50 mol% lithium carbonate Li ₂ CO ₃ , 10 mol% as a raw material.
Powders of tantalum pentoxide Nb ₂ O ₅ and 40 mol% of vanadium pentoxide V ₂ O ₅ are weighed and mixed, put in a platinum crucible and melted at 900 ° C.
The temperature is maintained at 200 ° C. for 10 hours to make it uniform.

Next, this melt was cooled to 930 ° C. at a cooling rate of 30 ° C./h, and the substrate shown in FIG. Then, the sample is placed in an electric furnace at 30 ° C.
The LiNbO ₃ thin film 52 shown in FIG. 6G was grown by gradually cooling to room temperature at a cooling rate of / h.

When the film thickness of the sample was measured, it was 25 μm in the portion grown on the domain-inverted portion 56 in the substrate 51 and 30 μm in the other portions.

In addition to vanadium pentoxide, boron oxide trioxide, lithium fluoride, potassium fluoride, boron trioxide and molybdenum trioxide, boron trioxide and tungsten trioxide, etc. are used as the flack material in the epitaxial growth of the thin film 52. May be used.

It is also possible to apply an electric field during the epitaxial growth of the thin film 52 to control the height of the thin film grown on both polarized portions.

When the domains of the thin film 52 and the substrate 51 were etched with an etching solution of nitric acid: hydrofluoric acid = 1: 1, the polarization directions of the polarization parts 54 and 55 were the same regardless of the period Λ. And the interface was almost perpendicular to the interface between the thin film 52 and the substrate 51 (FIG. 3 (g)).

Finally, the sample was divided into single domains by poling.

Al was sputtered on both surfaces of the sample to form electrodes, and DC of 10,000 V was applied between the electrodes at room temperature.

Next, when the sample was etched with the above-mentioned etching solution, the same figure (h) was obtained regardless of the period Λ.
A single-domain diffraction grating shown in was obtained.

[0030]

According to the present invention, it is possible to provide a method for manufacturing a fine ferroelectric optical waveguide type diffraction grating which couples a waveguide mode and a radiation mode with high efficiency with a small number of manufacturing steps.

[Brief description of drawings]

FIG. 1 is a sectional view of a ferroelectric diffraction grating according to the present invention.

FIG. 2 is a sectional view of a conventional ferroelectric diffraction grating.

FIG. 3 is a manufacturing process diagram of a ferroelectric diffraction grating of the present invention.

[Explanation of symbols]

51 ... Substrate, 52 ... Thin film, 56 ... Polarization inversion part, 81 ... T
i film, 82 ... Photoresist.

Claims (5)

[Claims] 1. A waveguide type diffraction grating, wherein a ferroelectric optical waveguide type diffraction grating having a concavo-convex portion on its surface uses a ferroelectric substrate whose polarization is inverted beforehand in a desired diffraction grating pattern. Manufacturing method. 2. The method of manufacturing a waveguide type diffraction grating according to claim 1, including a liquid phase epitaxial growth method. 3. The method of manufacturing a waveguide diffraction grating according to claim 1, wherein an electric field is applied during the liquid phase epitaxial growth method. 4. The ferroelectric optical waveguide type diffraction grating according to claim 1, 2 or 3, wherein lithium niobate (LiN
bO ₃ ), a method of manufacturing a waveguide diffraction grating. 5. The flux used in the liquid phase epitaxial method according to claim 2, 3 or 4, wherein vanadium pentoxide is used.
(V ₂ O ₅ ), boron trioxide (B ₂ O ₃ ), lithium fluoride (LiF), potassium fluoride (KF),
Or boron trioxide (B ₂ O ₅ ) and molybdenum trioxide (M
oO ₃ ), or a method for producing a waveguide diffraction grating containing boron trioxide (B ₂ O ₃ ) and tungsten trioxide (WO ₃ ).