JP3222092B2 - Manufacturing method of ridge structure optical waveguide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a ridge-structured optical waveguide. More specifically, the present invention relates to a method for manufacturing an optical waveguide used for a light control element or the like that performs light wave modulation, optical path switching, and the like.

[0002]

2. Description of the Related Art As a Mach-Zehnder type Ti heat diffusion LiNbO ₃ optical modulator capable of performing wideband optical modulation using a ridge-structured optical waveguide, a speed-matched light shown in FIGS. 2 (a) and 2 (b). Modulator (K. Noguchi, et al., "A Broadband T
i: LiNb03 optical modulatorwith a ridge structure, "
IEEE Journal of Lightwave Technology, Vol. LT-13,
pp.1164-1168,1995). FIG. 2A is a plan view and FIG.
(B) is a cross-sectional view along the line AA '.

In this example, a Mach-Zehnder optical waveguide 102 is formed on a z-cut LiNbO ₃ substrate 101 having an electro-optical effect by Li thermal diffusion. The substrate 101
An SiO ₂ buffer layer 103 is formed on the buffer layer 103, and a center conductor (center electrode) 104 is further formed on the buffer layer 103.
Further, a coplanar waveguide (CPW) type traveling wave electrode composed of an earth conductor (earth electrode) 105 is formed.

[0006] Reference numeral 106 denotes a terminating resistor connected between the traveling wave electrodes 104 and 105, and 107 denotes a traveling wave electrode 104 or 1.
A modulation microwave signal power supply line (power supply coaxial line) connected to the electrodes 105 and 105 for supplying a modulation microwave signal to the electrodes 104 and 105. Further, as shown in FIG. 2B, the substrate portion in the region where the microwave electric field strength is high near the center conductor 104 and the ground conductor 105 constituting the CPW electrode is dug down by etching or the like to form a ridge structure. I have.

That is, the substrate portion in a region where the microwave electric field strength is strong is formed into a protruding shape by digging down by etching or the like, and the buffer layer 103 is disposed on the surface of the substrate 101 on which the protruding portion 108 is formed. I do. At this time, the two optical waveguides 102
One of the central conductors 10 through the buffer layer 103.
4 and the other optical waveguide 102 is disposed directly below the ground conductor 105.

[0006] In the optical modulator shown in FIG.
By increasing the thickness of each of the layers 04 and 105 to some extent to lower the value of the microwave effective refractive index _nm , the characteristic impedance Z accompanying the decrease at that time is increased by making the ridge structure deeper to 3 μm to 4 μm. The impedance is thereby matched with the external circuit.

In particular, since the ridge structure is employed, the effective refractive index n _{m of the} microwave is also reduced. Therefore, the traveling wave electrode 104 and the traveling wave electrode 104 necessary to realize perfect velocity matching between the microwave and light are used. The thickness of 105 may be thinner than in the case where there is no digging. Therefore, traveling wave electrodes 104 and 1
A decrease in the characteristic impedance Z caused by increasing the thickness of the layer 05 can be suppressed, and
It is easy to manufacture the traveling wave electrodes 104 and 105, and furthermore, advantages such as suppression of increase in microwave propagation loss can be obtained.

The light control element shown in FIG. 2 is manufactured by, for example, the steps shown in FIG. FIG. 3 shows an example of a process of a conventional example of a method for manufacturing an optical waveguide having a ridge structure, using a cross-sectional view of a substrate. As shown in FIG.
An optical waveguide 202 is formed on an iNbO ₃ substrate 201.

First, as shown in FIG.
A Ti film 203 of about 0 nm is formed on the entire surface, and FIG.
As shown in FIG. 2, the resist pattern 204 having a groove that is to be dug down by a normal photo process is
It is formed on the i film 203. Subsequently, as shown in FIG. 3D, LiNb is used with the resist pattern 204 as a mask.
The Ti film 203 on the O ₃ substrate 201 is converted to CF ₄ gas plasma 2
Etching by a plasma etching method using 06 or the like,
By dissolving the photoresist, a Ti pattern 205 having the same shape as the resist pattern is obtained.

[0010] Thereafter, as shown in FIG.
By using the i-pattern 205 as a mask, the LiNbO ₃ substrate is etched by an ion beam etching method using an ion beam 207 of an argon gas and a fluorine-based gas, and the Ti film 203 is dissolved with hydrofluoric acid or the like, thereby obtaining FIG. A ridge-structured optical waveguide having a projection 208 as shown in FIG.

[0011]

In the conventional manufacturing method as described above, in order to efficiently etch the LiNbO ₃ substrate which is an oxide crystal, an ion beam etching method using argon or a fluorine-based gas is used as an etching method. In addition, a Ti pattern is used as an etching mask. The ion beam etching method using an argon or fluorine-based gas has a high etching rate for an oxide substrate.

Since Ti has high adhesion to the LiNbO ₃ substrate and has the second lowest sputtering rate for argon ions after carbon among all elements, the difference in etching rate with the LiNbO ₃ substrate can be increased. However,
The ridge-structured optical waveguide manufactured by the conventional manufacturing method has a problem that the propagation loss of the optical waveguide increases.

Further, in an optical control device using a ridge-structured optical waveguide manufactured by a conventional manufacturing method, a fluctuation of a DC bias point, that is, a so-called DC drift becomes large and operation becomes unstable. There was a problem. The present inventors have conducted intensive studies in order to solve the above problems, and as a result, have found that Ti, which is an etching mask, damages the substrate at the interface with the LiNbO ₃ substrate.

That is, Ti is an element having a very high oxygen adsorbing property. When Ti is deposited on a LiNbO ₃ substrate, it is strongly bound to oxygen atoms near the surface to form a Ti—O bond. When Ti is removed with hydrofluoric acid after the end of the etching process,
The oxygen atoms bonded to the Ti atoms are also removed. As a result, the ratio of oxygen atoms in the vicinity of the substrate surface decreases and L
It has been found that oxygen deficiency occurs in the iNbO ₃ substrate, which increases the propagation loss of the optical waveguide. Furthermore, the above LiNbO
_The inventors found that the oxygen deficiency of the _three substrates lowered the surface resistance and caused the DC drift of the light control element.

[0015]

SUMMARY OF THE INVENTION Based on the above findings,
Inventors have found that by pre-LiNbO ₃ thin film of oxide on the substrate, for example, silicon dioxide (SiO ₂₎ film is formed, to form a Ti film is an etching mask thereon, LiNbO ₃ substrate by Ti It has been found that oxygen vacancies near the surface can be eliminated. Accordingly, it is possible to suppress an increase in the propagation loss of the ridge-structured optical waveguide and to suppress the DC drift of the optical control element using the ridge-structured optical waveguide.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for manufacturing a ridge-structured optical waveguide according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram illustrating a method for manufacturing a light control element according to the present invention. As shown in FIG. 1A, the LiNbO ₃ substrate 30
The optical waveguide 302 is formed on 1.

First, as shown in FIG.
A SiO ₂ film 309 having a thickness of about 0 nm and a Ti film 303 having a thickness of about 500 nm are formed on the entire surface. Then, as shown in FIG. A pattern 304 is formed on the Ti film. Subsequently, as shown in FIG. 1D, the SiO ₂ film 309 and the Ti film 303 on the substrate 301 are etched by a plasma etching method using a CF ₄ gas plasma 306 or the like, and the photoresist is dissolved to form a resist. Ti pattern 305 and SiO ₂ having the same shape as the pattern
A pattern 310 is obtained.

[0018] Thereafter, as shown in FIG. 1 (e), it is etched by an ion beam etching method using a fluorine-based ion 307 described above, by dissolving the Ti film and the SiO ₂ film with hydrofluoric acid, Figure 1 A ridge structure optical waveguide having a protruding portion 308 as shown in FIG. Here, the SiO ₂ film 309 can be manufactured in the same batch just before the Ti film formation by the same method as the Ti film 303, for example, the electron beam evaporation method.

The etching of the SiO ₂ film 309 is also performed by T
CF ₄ gas plasma 30 simultaneously with etching of i-film 303
6 and can be simultaneously removed with hydrofluoric acid. That is, in the manufacturing method of the light control element of the present invention, the ridge-structured optical waveguide can be manufactured by using exactly the same apparatus and process as the conventional one, except that a process of forming the SiO ₂ film 309 is newly added.

In the above embodiment, a z-cut LiNbO ₃ substrate is used. However, an x-cut LiNbO ₃ substrate may be used, or another oxide substrate such as lithium tantalate (LiTaO ₃ ) may be used. Is also good. Further, as the oxide material of the first thin film, an aluminum oxide (Al ₂ O ₃ ) or titanium oxide (TiO ₂ ) thin film may be used, and as the material of the second thin film, Ta other than Ti may be used. For example, a thin film of an inorganic material such as a silicon nitride (Si ₃ N ₄ ) film, or a thin film of an organic material such as a polyimide film or a photoresist film may be used.

[0021]

As described above in detail with reference to the embodiments, according to the present invention, an oxide substrate (eg, LiNbO ₃ ) having at least one optical waveguide and a thickness of a part of the substrate are provided. In a method for manufacturing a ridge-structured optical waveguide in which the at least one optical waveguide is arranged in a protruding portion formed on the substrate by reducing the depth by digging, a thin film pattern formed on the oxide substrate is used as a mask, In the step of forming the thin film pattern in the step of manufacturing the dug portion, after forming an oxide material thin film (for example, SiO ₂ ) as a first thin film on the oxide substrate, the first thin film is formed. Since a second thin film (for example, Ti) is formed thereon, and then the thin film pattern is formed by a photo process, the propagation loss of the ridge-structured optical waveguide is suppressed, and at the same time, the ridge structure is reduced. In an optical control element using a di-structure optical waveguide, an element with a suppressed DC drift can be efficiently manufactured, so that not only the characteristics can be improved, but also the stability and mass productivity of the element can be improved. .

[Brief description of the drawings]

FIGS. 1A to 1E are process diagrams of one embodiment of a method for manufacturing a light control element of the present invention.

FIGS. 2A and 2B are a top view and a cross-sectional view of an example of a light control element having a ridge structure.

FIGS. 3A to 3E are process diagrams of a conventional method for manufacturing a light control element.

[Explanation of symbols]

101, 201, 301 LiNbO ₃ substrate 102, 202, 302 Ti thermal diffusion optical waveguide 103 Buffer layer 104, 105 Traveling wave electrode 106 Termination resistor 107 Feeding line 108, 208, 308 Projecting portion 203, 303 Ti film 204, 304 Photo resist 205, 305 Ti pattern 206, 306 CF ₄ gas plasma 207, 307 Argon and fluorine-based ion beam 309 SiO ₂ film 310 SiO ₂ pattern

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 6/12-6/14 G02F 1/00-1/125 G02F 1/29-7/00

Claims (4)

(57) [Claims] An oxide substrate provided with at least one optical waveguide, and said at least one optical waveguide is formed on a protrusion formed on said oxide substrate by reducing a thickness of a part of said substrate by digging down. In the method of manufacturing the arranged ridge structure optical waveguide, in the step of manufacturing the dug portion using the thin film pattern formed on the oxide substrate as a mask, the step of forming the thin film pattern includes the steps of: After forming an oxide material thin film as a first thin film, a second thin film is formed on the oxide material thin film, and then a thin film pattern is formed by photolithography.
And thereafter removing the first and second thin films . 2. The method for manufacturing a ridge-structured optical waveguide according to claim 1, wherein the oxide substrate is made of lithium niobate. 3. The method for manufacturing a ridge-structured optical waveguide according to claim 1, wherein the oxide material is silicon dioxide. 4. The method for manufacturing a ridge-structured optical waveguide according to claim 1, wherein said second thin film material is titanium.