JPS5911085B2 - Manufacturing method of optical modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a solid-state optical integrated circuit or an optical modulator, and particularly relates to a method for manufacturing a solid-state optical integrated circuit or an optical modulator, and particularly relates to an optical waveguide with a high refractive index formed on an optical single crystal substrate or a substrate made of an optical material such as glass. The present invention relates to a method for forming optical circuits and electrodes in layers.

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 Bell System Tec
hnicalJOumaF'VOl. 48, Taki 7, as a method for manufacturing an optical waveguide layer, a mask having an opening for an optical circuit pattern portion or a non-waveguide portion is made on a substrate with a low refractive index, and A diffusion source is deposited only in the wave portion and then diffused to form a high refractive index or low refractive index portion.Furthermore, only in the mask opening portion, implanted element ions are implanted by ion implantation to form a high refractive index portion. There is a method of forming a region of refractive index or low refractive index. The above methods are for forming optical waveguide parts locally inside the planar waveguide layer, but in contrast to these methods, there is a method of three-dimensionally processing the optical waveguide part of the planar optical waveguide layer using 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 forming method according to the present invention is related to the latter method mentioned 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 σ12, then Exp[1] (4
The power loss is proportional to πσ12C0Sθ1/λ1)2.

Here, θ1 is the reflection angle of the propagating light at the optical waveguide side surface, and λ1 is the wavelength of the propagating light. The height and width of the optical waveguide are generally rectangular, with a height and width of several microns or several tens of microns. In the chemical etching method, the etched portion is removed from the resist film by photoetching, and the material to be etched is immersed in a chemical etching solution to etch only the required portion, leaving only the optical circuit pattern mask portion unetched and exposed to light. This method takes advantage of the fact that a circuit is formed.

In general, this chemical etching method has the disadvantage that a phenomenon called side etching is noticeable, and etching progresses on the side surface 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. be. On the other hand, preferred methods for processing optical circuits 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. A metal mask, which has a lower etching speed than the substrate, is usually used as a pattern mask for ion etching. As the etching speed ratio increases, side etching is less likely to occur, and the cross-sectional shape of the waveguide also maintains a rectangular shape.
The conditions for the optical waveguide processing method are 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.

When a metal or oxide is directly deposited as a pattern mask on a conventional optical waveguide layer, the pattern mask is removed after etching using a chemical solvent, sputtering, etching, or other methods; For substrates that are unstable due to There are drawbacks that make it unapplicable.

Next, a rectangular optical waveguide is formed by the method described above, and when this is combined with, for example, a polarizing analyzer to make an optical modulator by utilizing the electro-optic effect, the width is several microns to several tens of microns and the depth is several microns. ~
The problem is how to form electrodes on an optical waveguide of several tens of microns, but it is difficult to form electrodes after a rectangular optical waveguide has already been formed using conventional methods for the following reasons.

First, when using the photo-etching method, the soft-off method and the method of creating the necessary openings by exposure can be considered, but it is impossible to uniformly apply resist to a surface that already has unevenness of several microns to several tens of microns. Therefore, accurate masking is difficult.

As a method other than photoetching, a method of attaching the electrode material using a metal mask having a predetermined opening may be considered, but it is difficult to manufacture an accurate metal mask with a width of several tens of microns or less due to processing accuracy. Even if masking is possible, a slight misalignment of the mask or the presence of a gap with the optical waveguide will cause the electrode material to wrap around the side surface of the waveguide, adversely affecting the operating characteristics. Even if a mask for electrode formation is made in advance, optical waveguides formed by chemical etching or ion beam etching will not necessarily have the desired waveguide width due to side etching effects, and as above, the wraparound of the electrode material may occur. This is extremely difficult due to the number of reasons such as

An object of the present invention is to first solve the above-mentioned difficulties in forming electrodes by forming electrodes in a predetermined shape before forming the above-mentioned rectangular waveguide, thereby accurately forming electrodes in arbitrary shapes, and In order to solve the above-mentioned drawbacks in forming optical circuits on the optical waveguide layer, we improved the method of forming the mask so that side etching does not occur and when removing the metal mask, the substrate and optical waveguide layer can be removed. An object of the present invention is to provide a method for forming an optical waveguide without damage.

The present invention provides a method for forming an optical circuit and an electrode on an optical waveguide layer formed on a substrate made of an optical single crystal or an optical material, in which an electrode material layer is attached to the surface of the optical waveguide layer,
After covering it with a first photoresist layer, the first photoresist surface is exposed to light through a predetermined electrode pattern mask and developed to form an electrode pattern of the first photoresist, followed by dry etching or etching. After etching the electrode material layer by chemical etching to form a predetermined electrode, the remaining first photoresist is chemically removed so that only the electrode exists on the surface of the waveguide layer. Then, a second photoresist layer, such as a metal such as titanium, chromium, or an oxide mask such as titanium oxide, silicon dioxide, aluminum oxide, etc., and a third photoresist layer are applied to the surface of the waveguide layer including the electrodes. After the sequential coating, the photoresist layer is exposed to light and developed, and the metal or oxide mask is subjected to dry etching (for example, ion etching, sputter etching, plasma etching, etc.) or chemical etching, starting from the surface layer. By creating a predetermined optical circuit pattern with stacked layers, and then dry etching the entire surface to form an optical circuit in the optical waveguide layer, and then removing the second photoresist, This is a method of manufacturing an optical modulator, characterized in that the multi-coating layer described above is removed.

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 sirenite group of bismuth oxide, gallium arsenide, gallium phosphide, etc., and optical material refers 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, for example, by epitaxial method,
It is formed by a diffusion method, an ion implantation method, or the like. The feature of the present invention that allows the above-mentioned electrode of any size to be formed accurately is that, for example, LiN
This is very useful when epitaxially growing bO3 and applying a voltage in the z direction to perform optical modulation.

Furthermore, as an example in which it is necessary to form electrodes on the back surface of the substrate and the top surface of the optical waveguide, there is an example in which a photoconductive material is used as the substrate material and a material that does not exhibit photoconductivity is used as the optical waveguide. An electrode is formed facing the optical waveguide, and a voltage is applied between the electrodes to guide the modulated light into the optical waveguide. Next, by irradiating the substrate with light that makes it a photoconductive material, the substrate layer has a low resistance, so most of the applied electric field is applied to the optical waveguide layer, making it possible to perform sufficient optical modulation with a low voltage. It is also possible to control the modulation voltage by light irradiation, and optical modulation by light is also possible.

Therefore, the use of a photoconductive material as the substrate material and the use of a material that exhibits almost no photoconductivity as the optical waveguide layer is based on this usefulness. Hereinafter, the present invention will be explained in detail by examples using the drawings. 1 and 2 are cross-sectional views showing a method for forming an electrode and a method for forming an optical circuit in an optical waveguide layer in the order of processing steps, respectively, in order to explain an embodiment of the method of the present invention. 1st
In Figure A, 1 is a substrate made of, for example, lithium tantalate single crystal, and 2 is an optical waveguide layer formed on the surface of which is made of lithium niobate, which has a different refractive index from that of the substrate 1, for example, has a higher refractive index. be. First, on the optical waveguide layer 2, as the electrode material 3, Au is vapor-deposited to a thickness of several hundred to several thousand λ, or Cr
After several hundred λ of Au is deposited, several hundred to several thousand λ of Au is deposited. Here, the reason why particularly Au or a composite layer of Cr and Au is used as the electrode material 3 is that electro-optic single crystals such as LiNbO3, LiTaO3, etc. are prepared using HCl, . When an electrode material that is chemically etched by an acid such as HNO3 or acetic acid is used, the substrate layer and optical waveguide layer are also etched when forming an electrode pattern. In addition, etching is also possible with an iodine solution (KI+2+H2O), and electro-optic single crystals such as LiNbO3 and LiTaO3 are hardly etched by this iodine solution. Y. Cr has good adhesion with oxides, and when Cr and then Au are deposited on the optical waveguide layer, Cr and A are bonded together.
Since the adhesion of u is also good, a strong electrode can be formed.
SO4)2 and/or Ce(SO4)2.2(NH4
)2. S04. A solution consisting of XH20}, like the iodine solution, has the advantage of hardly etching the substrate and waveguide layer. Next, a first photoresist 4 is applied on the Au or composite layer 3 of Cr and Au, and this is exposed through a predetermined electrode pattern mask and developed.
Form a mask pattern required as an electrode. In FIG. 1, 4 is a mask pattern made of the first photoresist formed in this way, and in this state, using the above-mentioned iodine solution and Cr etching solution, Au or Cr and A
A predetermined electrode 3 can be formed on the surface of the optical waveguide layer 2 by chemically etching the electrode material 3 of the composite layer of u. Next, a method for forming an optical circuit in the optical waveguide layer 2 will be explained with reference to FIG. First, a positive type second photoresist 5 is applied to a thickness of 2 to 3 μm on the surface of the waveguide layer 2 including the electrode 3 using a conventional spinner.

In this case, since the already formed electrode 3 has a thin thickness of several hundred to several thousand λ, it does not have any adverse effect on the application of the photoresist.
This is clear from the fact that the resolution did not deteriorate at all even when a pattern was formed using photoresist on a polished mirror surface with an unevenness of λ. A metal mask 6 (eg, titanium, chromium, etc.) or an oxide mask 6 (eg, titanium oxide, silicon dioxide, aluminum oxide, etc.) is deposited on the entire surface of the second photoresist 5 by sputtering.

Further, a positive or negative third photoresist 7 is applied to a thickness of 2 to 3 μm over the entire surface of the metal mask 6 (FIG. 1).
Next, the surface of the third photoresist 7 is exposed to light through a desired optical circuit pattern mask (not shown) by a normal light exposure method and developed, so that the third photoresist 7 is exposed as shown in the diagram.
An optical circuit pattern is formed.

Next, when the metal mask 6 is etched by chemical etching or sputter etching, as shown in Figure C, the remaining portion of the resist 7 is removed, and a metal pattern mask for optical circuits is completed.

In chemical etching, electro-optic single crystals such as LiNbO3 and LiTaO3 are etched using a solvent such as HF.
:HNO3=1:1 mixed solution, etching of the substrate layer can be prevented by applying a resist to the back and side surfaces of the substrate layer to an appropriate thickness.

When the patterned third photoresist 7 and metal mask 6 obtained in the above steps are used as a photomask and the surface is exposed again, the areas covered by the metal mask 6 are shielded from light, so during development. The photoresist 5 in that area is not dissolved and remains as shown in FIG.

Here, if the photoresist 7 is a positive type, it is removed during development, but if it is a negative type, it is not necessarily removed. Thereafter, when the entire surface is etched by ion etching, the optical circuit pattern portion is etched into layers 7, 6, and 5 sequentially from the top as shown in FIG.
In this case, the etching depth D of the material to be etched until the multi-coating layers 7, 6, and 5 are removed is determined by the following formula depending on the material and thickness of each coating layer and the material of the material to be etched (optical waveguide layer 2). Follow (1).

Here, Ds: Etching depth of the material to be etched Dml: Layer thickness of the third photoresist 7 Dm2: Layer thickness of the metal or oxide mask 6 Dnl3: Layer thickness of the second photoresist 5 Rs: Processing of the material to be etched Speed Rnll: Processing speed of the third photoresist 7 Rm2: Processing speed of the metal or oxide mask 6 Rnl3: Processing speed of the second photoresist 5 As described above, in the present invention, multilayers are laminated and used as a mask material. It can be seen that the mask effect is equivalent to that of a single mask.

In order to effectively exhibit the effects of the present invention, the planned etching depth of the material to be etched must be D8 given by equation (1).
It is necessary to select various etching conditions as smaller than .

Under such conditions, the photoresist 5 remains without being etched.

When a predetermined etching depth is obtained, a rectangular optical waveguide layer 2 is formed as shown in FIG.
Similarly, the electrode 3 of the u or Cr and Au composite layer remains, and the remaining mask pattern 5 or 5, 6 is removed by removing the second photoresist 5 with an effective remover. is removed as shown in the figure, and Au or Cr is precisely
A rectangular waveguide 2 having an At composite electrode 3 formed therein is obtained. In the present invention, by appropriately selecting the above-mentioned planned etching depth, the metal or oxide mask 6 can be easily removed during ion etching, and the photoresist 5 can be removed easily.
remains, so a highly accurate optical circuit pattern can be obtained without any damage to the substrate to be processed and without side etching.

Further, in the present invention, a second layer is provided between the mask 6 and the optical waveguide layer 2.
Since it has a photoresist 5, 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.

Finally, when the Au electrode 8 is formed on the back surface of the substrate by the usual photoetching method, for example, by combining it with a polarizing analyzer and applying a voltage under ultraviolet to blue light irradiation,
Optical modulators with precise waveguides and electrodes can be manufactured.

Next, embodiments of the present invention will be described.

EXAMPLE A LiNbO3 layer was epitaxially grown to a thickness of about 10 μm using a LiTaO2 Z plate as a substrate.

First, the surface of the LiNbO3 layer was optically polished to a plane roughness of λ/10 and a parallelism of less than 10 seconds so that an epitaxial layer of about 3 μm remained, and then C was polished by the method shown in FIG.
r to a thickness of about 400λ. After sequentially depositing Au photoresists with a thickness of approximately 2000λ and a thickness of 2μ each, an electrode pattern of the photoresists was formed by photoetching, K: 12: H2O = 60y: 20f: 100.
After etching Au for about 30 seconds with a room temperature iodine solution in Step 7, a room temperature Cr etching solution (H2SO4 and Ce(SO
4) Chemical etching was performed using an etching solution consisting of 2.2(NH4)2S04.2H20), and then the photoresist was removed to obtain a predetermined electrode pattern.

Thereafter, the optical waveguide layer was processed by the optical circuit forming method shown in FIG. The photoresists used were all positive type with a thickness of 2 μm, and Ti was deposited on the metal mask by high frequency sputtering. The thickness of each coating layer shown in FIG. 2A was as follows.

Second photoresist. 5 thickness 2 μ metal mask 6
Thickness of titanium: 0.3μ Thickness of third photoresist 7: 22 The width of the mask shown in Figures 1 to 5 was 3μ.

The ion etching processing conditions in the process shown in Figure E were as follows.

Accelerated ion realon acceleration voltage: 1000 Ion current: 1.077 LA/CrA Etching time: 120 minutes In formula (1), AZ-1 was used as the photoresist material.
When using 350 (trade name: manufactured by Shigure Ichi, USA, a mixture of naphthoquinone diazide thermal decomposition agent and alkali-soluble novolac resin), Dnll = 2X104λ, paste = 600
λ/min, Dm2=2×103λ, Rm2=200λ/min, and the etching speed of LiNbO3 is 270λ/min, so the etching depth after 120 minutes of etching is about 3μ.

As a result of this experiment, the etching depth was about 3 microns, almost as calculated above. After this ion etching, the remaining titanium metal mask was completely removed together with the photoresist as a result of removing it using a second photoresist material removal solution.

The obtained optical circuit was free from side etching and had high precision. Furthermore, the Cr and Au composite electrode 3 remained on the rectangular waveguide 2 with the same width, demonstrating the usefulness of the present invention.

As described above, the method of the present invention forms electrodes with a predetermined shape before forming an optical circuit on an optical waveguide.
In addition to being able to accurately form electrodes of arbitrary shapes, the surface of the optical waveguide layer including the electrodes is coated with three layers of photoresist, metal or oxide mask photoresist, and an optical circuit pattern is sequentially formed from above. Since the optical circuit is formed on the optical waveguide layer by ion etching, a highly accurate optical circuit pattern can be obtained, there is no damage to the substrate to be processed, there is no side etching, and there is no reaction with the substrate to be processed. It is possible to use a mask material having a certain property and to select an arbitrary mask material, which has the advantage that a high-quality optical modulator can be manufactured.

[Brief explanation of the drawing]

1, A to C, and FIG. 2, A to I are cross-sectional views showing a method for forming an electrode and a method for forming an optical circuit in an optical waveguide layer in the order of processing steps, respectively, in order to explain an embodiment of the method of the present invention. It is. 1... Substrate, 2... Optical waveguide layer, 3, 8
... Electrode (Au or Cr, 5Au composite layer),
4...First photoresist, 5...Second photoresist, 6...Metal or oxide mask, 7...Third photoresist.

Claims (1)

[Claims] 1. A method for forming an optical circuit and electrodes on an optical waveguide layer formed on a substrate made of an optical single crystal or an optical material,
After depositing an electrode material layer on the surface of the optical waveguide layer and covering it with a first photoresist layer, the first photoresist surface is exposed to light through a predetermined electrode pattern mask and developed. 1. By forming an electrode pattern of photoresist and etching the electrode material layer by dry etching or chemical etching,
After forming a predetermined electrode, the remaining first photoresist is chemically removed so that only the electrode exists on the surface of the waveguide layer, and then a second photoresist is formed on the surface of the waveguide layer including the electrode. After sequentially coating the second photoresist layer, the metal or oxide mask, and the third photoresist layer, the photoresist layer is exposed and developed, and the metal or oxide mask is subjected to dry etching or chemical etching, starting from the surface layer. A predetermined optical circuit pattern in which the three layers are stacked is created by applying the photoresist, and then dry etching is performed on the entire surface to form an optical circuit in the optical waveguide layer. A method of manufacturing an optical modulator, characterized in that the multi-coating layer is removed by removing. 2. The method of manufacturing an optical modulator according to claim 1, wherein the substrate is made of a photoconductive material and the optical waveguide layer is made of a material that exhibits almost no photoconductivity. 3. The electrode material is made of gold or a composite layer of chromium and gold, and an iodine solution consisting of potassium iodide, iodine, and water is used for the chemical etching of the gold, and a sulfuric acid-based chromium etching solution is used for the chemical etching of the chromium. A method of manufacturing an optical modulator according to claim 1. 4. The method for manufacturing an optical modulator according to claim 1, 2, or 3, wherein the metal mask is made of titanium or chromium, and the oxide mask is made of titanium oxide, silicon dioxide, or aluminum oxide. 5 Claim 1 wherein the dry etching is ion etching, sputter etching or plasma etching
4. A method for manufacturing an optical modulator according to item 2, item 3, or item 4. 6. Any one of claims 1 to 5, 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, which is given by the following formula: A method for manufacturing the optical modulator according to item 1. d_s=R_s(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 third photoresist d_m_2: Layer thickness of the metal or oxide mask d_m_3:
Layer thickness R_s of the second photoresist: Processing speed R_m_1 of the material to be etched: Processing speed R_m_2 of the third photoresist
: Processing speed of metal or oxide mask R_m_3: Processing speed of second photoresist.