JP2000221458A - Production of waveguide type optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process for producing a waveguide type optical device without damaging the waveguide formed on a crystal substrate or the substrate surface on the periphery of the waveguide. SOLUTION: This process consists in producing the waveguide type optical device of transverse electric field electrode constitution which has an optical waveguide part on the surface of the crystal substrate, is formed with transparent electrode films on both sides of the optical waveguide part or is formed with buffer films and is further formed with metallic films on these buffer films. In this process for producing the waveguide type optical device, the optical waveguide part formed on the surface of the crystal substrate is selectively protected by protective films and thereafter, the transparent electrode films or buffer films are formed by a sputtering method on the surface of the crystal substrate.

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 modulator and a waveguide type optical switch used in various systems such as high-speed optical communication, optical switching network, optical information processing and optical image processing. The present invention relates to an optical device manufacturing method.

[0002]

2. Description of the Related Art Waveguide-type optical modulators and waveguide-type optical switches are used for high-speed optical communication, optical switching networks,
This is a very important element in realizing various systems such as optical information processing and optical image processing. In these optical modulators or optical switches, among others, LiNbO
An optical device (hereinafter, LN device) using a ₃ (hereinafter abbreviated as LN) substrate is promising because of its small wavelength chirping at the time of modulation.

This LN optical device has a structure in which a low-loss strip waveguide is formed in a substrate by internal diffusion of titanium into LN, and an electrode is formed thereon via a buffer film. The structure has an electro-optic effect and realizes the operation of a switch or a modulator.

FIG. 2 is a schematic view of a waveguide type optical device having a lateral electric field electrode structure in a case where there is one optical waveguide.

FIG. 2A is a perspective view of a waveguide type optical device having a lateral electric field electrode structure, and electrodes 203 (signal electrode and ground electrode) are grounded on both sides of a waveguide 202 provided on an LN substrate 201. And has a lateral electric field (e.g., applied to x-cut or y-cut crystals).

A waveguide type optical device of this type is introduced in the following cited reference-1. Reference 1: "Waveguide Electrooptic Modulator"
s (guided electro-optic modulator) (IEEE T
ransactions on Microwave Theory and Technique
s, MTT-30, Vol. 8, No. 8 (1982), pp. 1121-1137).

[0007] The bandwidth of the waveguide type optical modulator mainly depends on the type, material and arrangement of the electrodes and the dielectric constant of the substrate. For a wide band, the traveling wave electrode
de) is widely used. The concept is to configure the electrode as an extension of the transmission line. Therefore, the characteristic impedance of the electrode depends on the microwave power source and the load (L
od). The modulation speed in that case is limited by the difference in transit time (or phase speed or effective refractive index) of the light wave and the microwave.

[0008] Traveling wave electrode structures widely used include: 1) an asymmetric strip line (hereinafter abbreviated as ASL) type comprising one signal electrode and one ground electrode, or an asymmetric planar strip (hereinafter abbreviated as ASL). Asymmetric coplanar strip (hereinafter abbreviated as ACPS) type electrode structure. 2) Coplanar waveguide consisting of one signal electrode and two ground electrodes,
There are two types of electrode structures, abbreviated as CPW).

These contents are introduced in the following cited reference-2. Cited Reference-2: "A wide band Ti: LiNbO ₃ optical
modulator with a conventional coplanar wavegu
ide type Electrode (with a conventional planar waveguide type electrode, broadband Ti: LiNbO ₃ modulators) entitled "Article (IEEE Photonics Technology Letters, Vol. 4 No. 9
No. (1992), pp. 1020-1022) FIG. 3 is an example of a lateral electric field electrode configuration having two optical waveguides on which ordinary or planar waveguide traveling wave electrodes are formed. The two waveguides are modulators or optical switches (directional couplers,
So-called directional coupler) two arms or Ma
It is represented by two arms of a ch-Zehnder modulator. FIG.
(A) is a perspective view, which has two waveguides 302 on the surface of an LN substrate 301, electrodes 303 formed on both sides thereof, and has an electrode configuration having a lateral electric field.

The structure of the above waveguide type optical device will be described with reference to FIGS. 2 (b) to 2 (d) and FIGS. 3 (b) to 3 (d). This will be described with reference to FIG.

FIGS. 2 (b), 2 (c) and 2 (d)
2A is a cross-sectional view cut along the line AB shown in FIG. 2A, and shows a variation of the cross-sectional structure of the waveguide optical device having the in-plane switching electrode configuration of FIG. 2A. The electrodes 203 formed on both sides of the waveguide 202 are, for example, metal electrodes or transparent electrodes.

FIG. 2B shows a case where the electrode 203 is directly formed on the LN substrate 201. FIGS. 2C and 2D show a case where the electrode 203 is formed on the LN substrate 201 via a buffer film. In this case, the electrodes are formed. The buffer film 204 shown in FIG. 2C is formed only under the electrode, and the buffer film 204 shown in FIG.
Buffer film 204 is also formed outside the electrode.

FIGS. 3 (b), 3 (c) and 3 (d)
3A is a cross-sectional view cut along a line AB shown in FIG. 3A, and shows a variation of the cross-sectional structure in the waveguide-type optical device having the horizontal electrode configuration of FIG. 3A.
Two waveguides 302 are formed on the LN substrate 301, and a lateral electric field electrode configuration is formed in which electrodes 303 are formed on both sides of the waveguide 302.

[0014] As shown in FIG.
In the case where the electrode 203 is directly formed on the N substrate 201, there are cases where the electrode is formed on the LN substrate 201 via a buffer film in FIGS. 3C and 3D. The buffer film 304 in FIG. 3C is formed only under the electrode, and the buffer film 304 in FIG. 3D is also formed outside the electrode.

As described above, FIG. 2A or FIG.
In the case of the horizontal electric field configuration shown in (1), for example, a modulator can be exemplified, but a buffer film below the electrode may be used for phase matching between light and microwaves.

FIG. 4 shows a method of manufacturing a waveguide type optical device having a lateral electric field electrode configuration having one optical waveguide shown in FIG. 2C. The manufacturing process is the same for the optical device having two optical waveguides shown in FIG.

A crystal substrate (for example, L
A waveguide 402 is formed on an (N substrate) 401 (step 1). The waveguide 402 is formed by forming a titanium metal film strip and then diffusing it into the crystal.

A buffer film or a transparent electrode film (ITO) 403 as a dielectric layer is formed thereon by a sputtering method (step 2).

A photoresist (PR) 404 is formed on the buffer film 403, and patterning (exposure, development, etc.) is performed (steps 3 and 4).

Next, the buffer film is etched using the patterned resist as a mask. The etching is performed by, for example, dry etching using an ECR apparatus or an RIE apparatus or wet etching using an etchant (Step 5).

Thereafter, the photoresist is removed (step 6).

On the substrate on which the buffer film or the transparent electrode is formed in a predetermined shape, a metal film 5 is formed as required (Step 7).

Next, a new photoresist 404 is formed on the metal film 405 (Step 8), the photoresist 404 is patterned (Step 9), and the metal film 405 is formed into a predetermined shape by dry etching or wet etching. Processing (step 10).

The photoresist 404 remaining on the processed metal film 405 is removed to complete the device (step 11).

Of the above steps, when forming the buffer film or the transparent electrode film 403 in step 4, by using a sputtering method, it is possible to obtain a film having good adhesion to the substrate and good quality. In that case, the waveguide 2 and the crystal substrate surface around the waveguide 2 (optical waveguide portion) are damaged by being exposed to a sputtering atmosphere. This damage easily causes a problem such as a defect or a decrease in resistance in the crystal, thereby deteriorating the reliability of the device (such as high DC drift and a decrease in temperature characteristics). This deterioration in reliability is described in the following references 3 and 4. Reference-3: "Ultra-broad-band and highly stab
le LiNbO3 optical modulators for optical tra
nsmission systems, IOOC'95
(International Conference on Integrated Opt
ics and OpticalFibre Communication) Technical Digest Volume 2, pages 100-101. Reference-4: "Highly Reliable LiN with Si Double Slit
bO ₃ Optical Modulator ”IEICE Technical Report, {Mechanical Devices} August 28, 1998, EM
D98-52, pp. 61-66).

[0026]

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present invention provides a method of manufacturing a waveguide type optical device without damaging a waveguide formed on a crystal substrate or a substrate surface around the waveguide. The aim is to provide a method.

[0027]

According to the present invention, an optical waveguide is provided on a surface of a crystal substrate, and a transparent electrode film or a buffer film is formed on both sides of the optical waveguide. A method for manufacturing a waveguide-type optical device having a lateral electric field electrode structure in which a metal film is formed on the buffer film, wherein the optical waveguide portion formed on the surface of the crystal substrate is selectively formed by a protective film. And then forming the transparent electrode film or the buffer film on the surface of the crystal substrate by a sputtering method.

The protective film can be formed without damaging the optical waveguide by performing an electron beam evaporation method or a CVD method.

The protective film is made of Ti, Au, A
1, Cr, W, Mo, WSi ₂ , MoSi ₂ , TiS
i ₂ , TaSi ₂ , TiN, TiW, SiO ₂ , In ₂ O ₃
Doped with SiO ₂ , PSG, BPSG or Si ₃ N
Preferably, the film is composed of _four .

Further, it is preferable that the transparent electrode is an ITO film or a zinc oxide film.

As described above, according to the present invention, after forming a waveguide and before forming a transparent electrode or a buffer film by a sputtering method, the substrate electron beam is prevented from damaging a portion of the substrate between the waveguide and the electrode. This is a method of manufacturing a waveguide-type optical device in which a protective film (for example, a metal, Ti, titanium film, or the like) is formed using an evaporation method, and a transparent electrode or a buffer film is formed by a normal sputtering method.

As described above, especially when the transparent electrode film or the buffer film is formed by sputtering, by protecting the optical waveguide, problems such as defects in crystals due to sputtering and reduction of resistance do not occur. Reliability can be ensured.

[0033]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view showing the steps of a basic waveguide type optical device according to the present invention.

A crystal substrate having an electro-optical effect (for example, L
A waveguide 102 is formed on an (N substrate) 101 (step 1). The waveguide 102 is formed by forming a titanium metal film strip and then diffusing the strip into a crystal.

A protective film 106 is formed on the substrate (Step 2). As for the type of the protective film, a metal film or an insulating film which can be formed by using an electron beam evaporation method or a CVD method without damaging the optical waveguide portion can be used.

For example, Ti, Au, Al, Cr, W, M
o, WSi_Two, MoSi_Two, TiSi _Two, TaSi_Two, Ti
Metal film of N, TiW, etc., SiO_Two, In_TwoO_ThreeDope
SiO_Two, PSG (Phosposilicate glass), BPS
G or Si_ThreeN_FourFilm consisting of
The optical waveguide is formed by using the sub-beam evaporation method.
It can be formed without damaging the part. FIG.
It is the figure which modeled the principle of the child beam evaporation method. Crucible 5
01 to the protective film raw material 502 held in
Vaporized by irradiating the
In this method, the plate 503 is exposed to form a protective film 504.

Of the films listed above, Al, W,
Mo, WSi ₂ , MoSi ₂ , TiSi ₂ , TaSi ₂ , S
The film using iO ₂ , PSG, BPSG or Si ₃ N ₄ can be formed by a CVD method. In this case, as in the case where the electron beam evaporation method is used, the optical waveguide portion is not damaged.

Furthermore, since the buffer film and the protective film need to be separately etched in different steps, it is preferable that the buffer film and the protective film are formed of different films. For example, when the buffer film is SiO ₂ , it is preferable to use a film other than SiO ₂ as the protective film.

The thickness of these protective films is several hundreds
Alternatively, by forming the substrate to a thickness larger than that, even if the substrate is exposed to a sputtering atmosphere in a later step, the optical waveguide portion is not damaged.

Next, a photoresist (PR) 104 is formed on the entire surface of the protective film 106, and then patterned (exposure, development, etc.) to leave the photoresist (PR) 104 only at a desired position. (Step 3).

Next, the remaining photoresist 104
Is used as a mask to leave the protective film 106 on the optical waveguide (step 4).

Thereafter, a buffer film or a transparent electrode film (hereinafter referred to as a buffer film) 103 is formed by a sputtering method (step 5). At this time, the optical waveguide portion is not damaged by the sputtering method since it is protected by the protective film. For transparent electrode films, ITO
A film or a zinc oxide film can be given as a preferable example. As for the buffer film, SiO ₂ film, Al ₂
An O ₃ film can be mentioned as a preferable example.

Next, in order to remove a portion of the buffer film or the like 103 formed on the protective film 106,
A photoresist (PR) 104 is formed at desired positions (step 6).

Next, of the buffer film 103 or the like, a portion formed on the protective film 106 is removed by a dry etching process using an ECR device or an RIE device or a wet etching process using an etching solution. The photoresist 104 is removed (Step 7). As a result, a buffer film or the like 103 is formed at a desired position other than the optical waveguide.

Next, a metal film 105 is formed, and a photoresist 104 is formed (Step 8).

Next, to leave a metal film at a desired position,
The photoresist 104 is patterned (exposed and developed) (step 9).

Next, using the photoresist 104 as a mask, a part of the metal film 105 and a part of the buffer film 103 are removed by a dry etching process or a wet etching process (Step 10).

Next, the photoresist 104 used as a mask is removed (step 11).

Next, the protective film 106 remaining on the optical waveguide portion
Is removed, and a final waveguide type optical device is completed (Step 12). The protective film is removed by wet etching so as not to damage the optical waveguide during the removal.

In some cases, a protective film may be left if necessary in design. In such a case, a waveguide type optical device is completed after step 11.

In the above-described steps, when a transparent electrode film is formed, a metal electrode film is further formed, but only a transparent electrode may be used. In that case, steps 1 to 7 are performed in the same manner, and after forming a transparent electrode film at a desired position, further proceeding to step 12 and removing the protective film, the waveguide type optical device in the final form is obtained. Can be formed.

[0052]

Until now, in the case of a device having a lateral electric field electrode structure, the upper surface of the waveguide and the upper surface of the substrate between the electrodes are damaged by sputtering, thereby reducing defects and resistance in the crystal. And other problems are likely to occur. As a result, the reliability of the element (high DC drift occurrence, deterioration of temperature characteristics, etc.) deteriorates. The present invention is a new manufacturing method using a protective film, which can prevent spatter damage on a portion of a substrate between a waveguide and an electrode and can secure the reliability of an element.

[Brief description of the drawings]

FIG. 1 is a process sectional view of a basic waveguide type optical device according to an embodiment of the present invention.

FIG. 2 shows the structure of an optical device using a lateral electric field electrode configuration having one optical waveguide. FIG. 2A shows a perspective view, and FIGS. 2B to 2D show A-
It is sectional drawing which cut | disconnected the cross section between B.

FIG. 3 shows the structure of an optical device using a lateral electric field electrode configuration having two optical waveguides. FIG. 3A shows a perspective view, and FIGS. 3B to 3D show A-
It is sectional drawing which cut | disconnected the cross section between B.

FIG. 4 is a process sectional view of a waveguide type optical device using a conventional technique.

FIG. 5 is a schematic diagram for explaining the principle of an electron beam evaporation method used when forming a protective film in the present invention.

[Explanation of symbols]

 101, 401 Crystal substrate having electro-optic effect 102, 402 Optical waveguide 103, 403 Buffer film or transparent electrode film 104, 404 Photoresist (PR) 105, 405 Metal film 106 Protective film 201, 301 Crystal substrate having electro-optic effect 202, 302 Optical waveguide 203, 303 Electrode 204, 304 Buffer film 501 Crucible 502 Material for protective film 503 Substrate 504 Protective film

Claims (6)

[Claims] An optical waveguide portion is provided on a surface of a crystal substrate, and a transparent electrode film is formed on both sides of the optical waveguide portion.
Alternatively, a method for manufacturing a waveguide-type optical device having a lateral electric field electrode structure, in which a buffer film is formed and a metal film is formed on the buffer film, wherein the optical waveguide formed on the surface of the crystal substrate Wherein the transparent electrode film or the buffer film is formed on the surface of the crystal substrate by a sputtering method. 2. The method according to claim 1, wherein the protection film is formed by an electron beam evaporation method or a CVD method. 3. The method according to claim 1, wherein the protective film is made of Ti, Au, Al, C
r, W, Mo, WSi _Two, MoSi_Two, TiSi_Two, Ta
Si_Two, TiN, TiW, SiO_Two, In_TwoO_ThreeDope
SiO_Two, PSG, BPSG or Si_ThreeN_FourConsists of
3. The optical waveguide device according to claim 2, wherein the optical waveguide is a film.
Vice manufacturing method. 4. The method for manufacturing a waveguide-type optical device according to claim 1, wherein said transparent electrode film is an ITO film or a zinc oxide film. 5. The method according to claim 1, wherein the buffer film is an SiO ₂ film, Al ₂ O
_The method for manufacturing a waveguide-type optical device according to any one of claims 1 to 4, wherein the film comprises _three films. 6. The method according to claim 1, wherein the crystal substrate is LiNbO._Three, LiT
aO_Two, KTiOPO _Four2. The method according to claim 1, wherein
6. A method of manufacturing a waveguide-type optical device according to any one of items 1 to 5,
Law.