JPS6032844B2 - Method of manufacturing optical waveguide - Google Patents

Description

Detailed Description of the Invention The present invention relates to the manufacture of solid optical waveguides such as optical integrated circuits, optical modulators, optical switches, optical directional couplers, etc. The present invention relates to a method for creating an optical circuit in a high refractive index optical waveguide layer formed thereon.

Conventionally, there have been various methods for creating an optical circuit using an optical single crystal or other material as a substrate and using a planar optical waveguide layer formed on the substrate to further limit the optical path to a specific region within the plane. It was suggested to me.

For example, in 1969 “The BellS disaster temTech”
icaIJoumarVol. 48, M. 7, as a method for manufacturing an optical waveguide layer, a mask having a gate for an optical circuit pattern portion or a non-waveguide portion is made on a substrate with a low refractive index, and a mask is formed on a substrate having a low refractive index. only,
A method in which a diffusion source is evaporated and then diffused to form a high refractive index or low refractive index region, and further, implanted element ions are implanted only in the mask closed region,
There is a method of forming a region with a high refractive index or a low refractive index. The above methods are for forming an optical waveguide portion locally inside a planar waveguide layer, but in contrast to these methods, there is a method of three-dimensionally processing the optical waveguide portion of a planar optical waveguide layer by an etching method. In this case, a three-dimensional optical waveguide can be formed by selecting an etching method and a mask material suitable for the material to be etched, and etching the area other than the part covered with the mask material in the form of an optical circuit pattern. is possible.

The optical waveguide manufacturing method according to the present invention is related to the latter method described above. Etching of the optical waveguide requires high-precision processing, and the cross-sectional shape of the waveguide and the surface accuracy of the side surface of the waveguide must be on the order of the wavelength or less. For example, if the surface roughness is 0.2, then the exp ．． 〔one(
4m.

,2coso,/in, )2 The power loss is proportional to . However, a, is the reflection angle of the propagating light at the side of the optical waveguide, ^
, is the wavelength of the propagating light. The height and inside of the optical waveguide are generally rectangular, with a height of several meters or several tens of micrometers.
In the chemical etching method, the etched part is removed from the resist film by photoetching, and the material to be etched is immersed in a chemical etching solution, so that only the required part is etched, leaving only the optical circuit pattern mask part unetched. This method takes advantage of the fact that an optical circuit is formed.

In this general chemical etching method, a phenomenon called side etching is noticeable, and the etching progresses on the sides and lower part directly below the resist film, and the cross-sectional shape of the waveguide tends to deviate from a rectangle and become trapezoidal or triangular. There is. On the other hand, preferred methods for optical circuit processing include dry etching, which is less likely to cause side etching, such as ion etching, sputter etching, and plasma etching.

In particular, ion etching has the advantage that side etching is less likely to occur because the accelerated ions involved in etching have good directivity. Usually, a metal mask whose etching rate is lower than that of the substrate is used as a pattern mask for ion etching. As the etching rate ratio increases, side etching is less likely to occur, the cross-sectional shape of the waveguide also maintains a rectangular shape, and the conditions for the optical waveguide processing method become appropriate. This pattern mask is generally coated onto the workpiece by vapor deposition or sputtering according to an etching or lift-off method. In this case, it is usually necessary to remove the mask after forming a predetermined optical waveguide.

For example, when passing propagating light through an optical waveguide circuit, the so-called evanescent mode light waves that exist outside the waveguide part increase the propagation loss in the part where the metal mask is in close contact with the optical waveguide. It is preferable to remove the mask after etching. In a typical conventional method, ion etching is performed by directly coating a metal mask on a substrate layer, so removing unnecessary metal masks after etching is performed using chemical solvents or physical methods such as sputter etching. However, these methods use chemically unstable substrates, such as bismuth silicon oxide or bismuth germanium oxide single crystals, which are non-corrosive resistant to strong acids, and substrates that react with mask removal solvents, such as niobic acid, which reacts with hydrogen fluoride. , there is a drawback that it cannot be applied to lithium single crystals, etc. In order to solve the above-mentioned drawbacks, it is an object of the present invention to manufacture an optical waveguide that does not cause side etching and does not damage the substrate or optical waveguide layer when removing the metal mask by improving the mask forming method. The purpose is to provide a method.

In order to achieve the above object, the present invention provides a method for creating an optical circuit in an optical waveguide layer formed on a substrate made of an optical single crystal or an optical material, in which a first photoresist layer is formed on the surface of the optical waveguide layer. , a metal such as titanium, chromium, or an oxide mask such as titanium oxide, silicon dioxide, aluminum oxide, etc., and a second photoresist layer, and then the second photoresist layer is first applied through the optical circuit pattern mask.
The second photoresist surface is exposed and developed.
Form a photoresist optical circuit pattern and perform dry etching (e.g. ion etching, sputter etching, etc.)
forming an optical circuit pattern in the mask by etching the mask by plasma etching or chemical etching, then exposing the first photoresist surface through the patterned second photoresist and the mask and developing; By doing so, an optical circuit pattern is formed on the first photoresist, and then dry etching is performed on the entire surface to form an optical circuit on the optical waveguide layer. This method of manufacturing an optical waveguide is characterized in that the multi-coating layer is removed by removing it with an effective solvent.

In the present invention, the optical single crystal that serves as a substrate is a material that has optical properties such as electro-optic effect, piezoelectric effect, nonlinear effect, and laser host, such as lithium niobate, lithium tantalate, bismuth silicon oxide, and bismuth germanium oxide. It refers to single crystals such as the sirenite group of bismuth oxide, gallium phosphorus, gallium phosphide, etc., and optical materials refer to transparent materials such as glass. The optical waveguide layer formed on these substrates is a thin film layer having a higher refractive index than the substrate, and is formed by, for example, an epitaxial method, a diffusion method, an ion implantation method, or the like. Hereinafter, the present invention will be explained in detail by examples using the drawings.

FIGS. 1A to 1A are cross-sectional views shown in order of processing steps to explain an embodiment of the method of the present invention.

In the figure, 1 is a substrate made of, for example, a lithium tantalate single crystal, and 2 is a lithium niobate single crystal formed on the substrate 1 by epitaxial growth and has a different refractive index than the substrate 1, for example, a higher refractive index. This is an optical waveguide layer consisting of.

The figure shows the case where an optical circuit is formed in this optical waveguide layer 2.

First, a negative type photoresist 3 is applied to the surface of the optical waveguide layer 2 to be processed to a thickness of 2 to 3 peaks using an ordinary spinner.

A metal mask 4 (for example, titanium, chromium, etc.) or an oxide mask (for example, titanium oxide, silicon dioxide, aluminum oxide, etc.) is deposited on the entire surface by sputtering. Further, a negative type photoresist 5 is applied to a thickness of 2 to 3 mm over the entire surface of the metal mask 4. (Figure A). Next, a desired optical circuit pattern mask (
When the surface of the photoresist 5 is exposed to light through a photoresist (not shown) and developed, an optical circuit pattern of the photoresist 5 is formed as shown in the diagram. Next, when the metal mask 4 is etched by chemical etching or sputter etching,
As shown in Figure C, the remaining portion of the resist 5 is removed, and a metal pattern mask for an optical circuit is completed. When the patterned photoresist 5 and metal mask 4 obtained in the above steps are used as a photomask and the surface is exposed again,
Since the area covered by the metal mask 4 is shielded from light, the photoresist 3 in that area will not dissolve during development.
As shown in Figure 2, it remains.

Thereafter, when the entire surface is etched by ion etching, the optical circuit pattern portion is etched into layers 5, 4, and 3 sequentially from the top as shown in FIG.

In this case, the etching depth ds of the material to be etched until the multi-coating layers 5, 4, and 3 are removed depends on the material and thickness of each coating layer and the material of the material to be etched (optical waveguide layer 2) according to the following formula m.

dS=RS (10 historical records). ...o' here, ds
: Etching depth dm of material to be etched: Resist 5
Layer thickness dm2: Layer thickness dm3 of metal or oxide mask 4: Shoulder thickness Rs of resist 3: Processing speed Rm of the material to be etched: Processing speed Rm2 of resist 5: Processing speed Rm3 of metal or oxide mask 4: Processing speed of resist 3 As described above, it can be seen that the present invention has a masking effect equivalent to that of a single mask even when multiple layers are stacked and used as a mask material.

In order to effectively exhibit the effects of the present invention, the planned etching depth of the material to be etched must be determined by d given by formula '11.
It is necessary to select various etching conditions with the value smaller than s.

Under such conditions, the photoresist 3 remains without being etched.

After obtaining a predetermined etching depth, the photoresist 3 is removed with an effective solvent, thereby removing the remainder of the multi-coat layer described above, as shown in FIG. In the present invention, by appropriately selecting the above-mentioned planned etching depth, the metal or oxide mask can be easily removed during ion etching, and since the photoresist 3 remains, the substrate to be processed is not damaged in any way. Additionally, there is no side etching and a highly accurate optical circuit pattern can be obtained.

Further, in the present invention, a first layer is provided between the mask 4 and the optical waveguide layer 2.
Since it has photoresist 3, it can be applied even when using a metal or oxide mask material that is reactive with the substrate to be processed.
It becomes possible to select any mask material. In general, many optical circuit substrates have a low etching speed, and in order to increase the etching depth, a metal or oxide mask material with a low etching speed Rm2 in formula 1 is required, but the above-mentioned It is a great advantage of the present invention that any material can be used as a mask.

Next, examples of the present invention will be described. Examples: In optical circuits, optical single crystals such as lithium niobate and lithium tantalate are used as substrate materials in thin film optical waveguides that are applied to optical modulators, optical switches, and the like.

In this case, a diffusion source is usually formed on a single crystal substrate by vapor deposition, and then diffusion is performed by heat treatment to form the above-mentioned planar optical waveguide layer, and then an optical waveguide is formed in a plane. The processing method of the invention was used. The processing steps were performed in the order shown in Figure 1A.

The thickness of each coating layer shown in Figure A was as follows. Thickness of the first photoresist 3 Thickness of titanium of the two-layer metal mask 4 Thickness of the second photoresist 5 0.2-layer Thickness of the mask shown in the diagrams 2 mm to 3 mm.

The ion etching processing conditions in steps not shown were as follows.

Accelerating ions: Argon Accelerating voltage: 1000V Ion current: 1. Etching time: 4 min or m type, AZ-135 was used as a photoresist material.
0 (product name: manufactured by Shigley, USA; mixture of naphthoquinone diazide thermal decomposition agent and alkali-soluble novolac resin), dm, = 2 x 1 and A, Rm = 600 A/min, dm2 = 2 x 1 Since the etching speed of lithium niobate is 270 A/min and the etching speed of lithium niobate is 270 A/min, the etching depth after 40 minutes of etching is about 1.

As a result of this experiment, the etching depth was about one peak, almost as calculated above. After this ion etching, the titanium metal mask was removed using a photoresist removal solution, and the titanium metal mask was almost completely removed together with the photoresist.

The obtained optical circuit had no side etching and had high precision. As described above, in the method of the present invention, three layers of photoresist metal or oxide mask photoresist are coated on the surface of the substrate, an optical circuit pattern is formed from above, and a photoconductor is etched by ion etching. Since the optical circuit is formed in the wave layer, a highly accurate optical circuit pattern can be obtained, and there is no damage to the substrate to be processed, there is no side etching, and a mask material that is reactive with the substrate to be processed is used. Any mask material can be selected.

Further, since the mask material remaining after ion etching can be easily removed together with the first photoresist, the present invention has the advantage that processing is extremely easy.

[Brief explanation of the drawing]

In order to explain the embodiment of the method of the present invention, FIG.
It is sectional drawing shown in the order of a processing process. 1...substrate, 2...optical waveguide layer, 3...
. . . first photoresist, 4 . . . metal or oxide mask, 5 . . . second photoresist. O diagram

Claims (1)

[Claims] 1. A method for creating an optical circuit in an optical waveguide layer formed on a substrate made of an optical single crystal or an optical material, wherein a first photoresist layer, a metal or oxide layer is provided on the surface of the optical waveguide layer. After sequentially coating the mask and the second photoresist layer, the second photoresist surface is first exposed through an optical circuit pattern mask and developed to form an optical circuit pattern of the second photoresist, and then dry etched or etched. forming an optical circuit pattern on the mask by etching the mask by chemical etching, and then exposing and developing the first photoresist surface through the patterned second photoresist and the mask; , by forming an optical circuit pattern on the first photoresist, and then dry etching the entire surface to form an optical circuit on the optical waveguide layer, and then removing the first photoresist. , a method for manufacturing an optical waveguide, comprising removing the multi-coating layer. 2. The method of manufacturing an optical waveguide according to claim 1, wherein the metal mask is made of titanium or chromium, and the oxide mask is made of titanium oxide, silicon dioxide, or aluminum oxide. 3
3. The method of manufacturing an optical waveguide according to claim 1, wherein the dry etching is ion etching, sputter etching or plasma etching. 4. The method of manufacturing an optical waveguide according to claim 1, wherein the final dry etching is performed by selecting etching conditions such that the planned etching depth of the optical waveguide layer is smaller than d_s given by the following equation. d_s=Rs((d_m_1)/(R_m_1)+(d
_m_2)/(R_m_2)+(d_m_3)/(R_
m_3)) However, d_s: Etching depth of the material to be etched until the multi-coating layer is removed d_m_1: Layer thickness of the second photoresist d_m_2: Layer thickness of the metal or oxide mask d_m_3:
Layer thickness Rs of the first photoresist: Machining speed R_m_1 of the material to be etched: Machining speed R_m_2 of the second photoresist
: Processing speed of metal or oxide mask R_m_3: Processing speed of first photoresist