JP3138549B2 - Method of fabricating ridge type on optical crystal substrate or thin film on the substrate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forming a ridge on at least a part of an optical single crystal substrate or a thin film formed on the substrate, and is particularly advantageously applied to manufacture a ridge optical waveguide. This is a proposal for the technology to be used.

[0002]

2. Description of the Related Art In order to fabricate a ridge type on an optical single crystal substrate or a thin film formed on the substrate, generally, Ta or A is used.
Step of forming a mask pattern using a metal such as l, Cr, Ti, or photoresist, RIE, RIB
E method or ion beam etching method
Usually, a step of performing dry etching is performed.

[0003] That is, the most common ridge type manufacturing method is as shown in FIG. 1, wherein an optical single crystal substrate 1 (FIG. 1 a) is used.
Alternatively, a photoresist 3 is applied to a thin film (not shown) formed on the substrate (FIG. 1b), and then exposed and developed with ultraviolet rays to form a resist pattern 3a (FIG. 1c). A metal film 2 of Al or the like is formed by a sputtering method or a sputtering method (FIG. 1d). Finally, the substrate is immersed in an organic solvent such as acetone or trichlene to form a metal foil of Al or the like thereon together with the resist pattern 3a. 2
This is a method of forming an Al metal pattern 2a by peeling off (FIG. 1e). This general method called a lift-off method consisting of such a series of steps involves two steps of pattern transfer, so that the steps become longer,
There is a problem that the accuracy of the so-called metal mask pattern is poor.

In order to solve such a problem, a method as described in Japanese Patent Application Laid-Open No. 54-158870 has been proposed. In this conventional method, Al is placed on an optical single crystal substrate 1.
A metal such as Al is formed by using a method such as a vacuum evaporation method or a sputtering method (FIG. 2B), and then a pattern of a photoresist 3 is formed on the metal 2 such as Al thus obtained (FIG. 2B). 2c) Finally, wet etching with an etchant is performed to leave the metal mask pattern 2a of Al or the like (FIG. 2d), and thereafter, the ridge portion 1a is formed using the metal mask pattern 2a as a mask. However, in the case of this conventional method employing wet etching, since this is an isotropic etching, the resist pattern is not accurately transferred, so that the accuracy of the metal mask pattern 2a is still poor. Was left.

In the conventional method proposed in Japanese Patent Application Laid-Open No. 4-104233, a photoresist film 3 is coated on an optical single crystal substrate 1 (FIG. 3B), and exposed and developed by ultraviolet rays to form a resist pattern. 3a is formed (FIG. 3
c), a ridge portion 1a is formed directly using this resist pattern as a mask (FIG. 3d). According to this conventional method, there is an advantage that there is no time waste in forming the mask pattern. However, in this case,
In the dry etching process by the RIE method, a reactive gas must be used, and this reactive gas easily reacts with a photoresist or an object to be etched, so that deposition (re-deposition) occurs. Was. Therefore, in this conventional method, it is necessary to sufficiently control the reaction gas pressure, the substrate temperature, the input power, and the like. In addition, when dry etching is performed by the RIE method in this method, it is difficult to control the resist etching rate because the etching rate of the resist greatly changes due to a slight difference in the reaction gas pressure.

Further, there is a method disclosed in Japanese Patent Application Laid-Open No. 54-6570. In this conventional method, LiNb
An O ₃ thin film 4 is formed, and a mask having a three-layer structure in which metal 2 is sandwiched between photoresists 3 and 3 ′ is used on the thin film 4 (FIG. 4).
a) Method, LiNb by ion beam etching
A ridge portion 1a of an O ₃ thin film is formed. This conventional method is different from the above-described RIE method in that dry etching is performed by ion beam etching. Therefore, the etching rate ratio between the resist and the optical single crystal substrate or the thin film on the substrate is good, and the etching rate of the resist is high. Is also stable. However, the ion beam etching employed under this conventional method has a problem that the etching rate ratio at the sample fixing position is not uniform due to the Gaussian distribution of the ion beam, so that it is difficult to form an arbitrary step. There was a point.

[0007]

As described above,
It has been found that both the RIE method and the ion beam etching method are not suitable as conventional dry etching methods for producing a ridge type on an optical single crystal substrate or a thin film formed on the substrate. In regard to such a dry etching method, the present inventors have paid attention to the plasma etching method and examined its application to the above-described problems of the conventional technology. In conducting this study, the inventors have found that when using the photoresist itself as a mask in order to improve pattern accuracy, there is a serious problem that the photoresist is ashed under conventional plasma etching conditions. I understood.

Therefore, various tests were conducted to solve such problems. As a result, the cause of this incineration is:
It was found that the oxygen partial pressure in the reaction zone was appropriate or inappropriate. That is, under the conventional conditions, the pressure before flowing the inert gas into the chamber is about 10 ^-2 to 10 ^-1 Pa. However, it has been found that at such a reduced pressure, the partial pressure of the oxygen remaining in the chamber is unexpectedly high, and therefore, ashing of the photoresist occurs. An object of the present invention is to solve the above-mentioned problems associated with the etching defects of the conventional technologies, to achieve a high uniformity of an etching rate ratio, a good pattern accuracy, and a low propagation loss ridge type. Is to provide a method for advantageously producing

[0009]

Means for Solving the Problems In order to realize the above-mentioned object, as a result of further intensive studies, the inventors have found that when a photoresist is directly used as a mask, an inert gas under a certain condition is obtained. It has been found that by performing dry etching by plasma, an etching rate ratio between the mask and the optical material is good, and a resist having a stable etching rate and high uniformity can be obtained.

That is, the above-mentioned certain condition is that dry etching is performed by inert gas plasma in a state where the partial pressure of oxygen remaining in the chamber is reduced.
In particular, in the present invention, a photoresist is applied on an optical single crystal substrate or a thin film formed on the substrate, a resist pattern is formed by exposure and development with ultraviolet light, and the resist pattern is used as a mask to be inert in a chamber. In a method of manufacturing a ridge type by performing dry etching by gas plasma,
This dry etching treatment is performed by reducing the pressure in the chamber before flowing an inert gas into the chamber to 10 ⁻² Pa.
And the pressure in the chamber after flowing an inert gas into the chamber is 7 × 10 ⁻² Pa or more,
This method is characterized in that a ridge type is formed on an optical crystal substrate or a thin film on the optical crystal substrate. In addition, as the inert gas, Ne, Ar, Kr,
Xe or the like can be used, but Ar gas is particularly preferable because it is inexpensive and easily available.

[0011]

According to the present invention, it is necessary that the pressure before flowing the inert gas into the chamber is less than 10 ^-2 Pa.
Preferably, the ^pressure is 10 ⁻³ Pa or less, and 10 ^{−7 to} 9 × 10
It is more preferable to control to ^-4 Pa. The reason for this is that at a pressure of 10 ^-2 Pa or more, the oxygen partial pressure in the chamber is higher than the level required by the present invention, so that the photoresist is ashed and the etching rate of the resist is increased. It is.

Next, the pressure in the chamber after flowing the inert gas into the chamber is 7 × 10 ^-2 P
It is necessary to be not less than a, preferably not more than 90 Pa, and more preferably 1 × 10 ⁻¹ Pa to ¹ Pa. The reason is that if the pressure is less than 7 × 10 ⁻² Pa, the partial pressure of the inert gas is too small to perform etching, and if it exceeds 90 Pa, it is difficult to generate plasma.

The substrate temperature during the etching is 25 to 13
Desirably, it is 0 ° C. The reason for this is that if the temperature exceeds 130 ° C., incineration is accelerated, whereas if it is less than 25 ° C., no plasma is generated.

In the present invention, the optical single crystal substrate to be used and the thin film formed on the substrate include a lithium niobate substrate and a lithium tantalate substrate, or a lithium niobate thin film formed on a lithium tantalate substrate. It is suitable. The reason for using these materials only is that these materials have large electro-optic constants and nonlinear optical constants, and are useful as optical devices. Note that these crystals may be subjected to treatment such as Ti diffusion and proton exchange.

The intensity of the inert gas plasma during the dry etching process is expressed as 1 × 10 ² to 1 × 10 ^{2 in} terms of power input per unit volume (area of electrode × distance between electrode and target).
1 × 10 ⁶ W / m ³ , more preferably 1 × 10 ⁴ to
It is 1 × 10 ⁵ W / m ³ . The reason for controlling the plasma intensity to such a level is that if the plasma intensity is less than 1 × 10 ² W / m ³ , the etching rate is reduced and the productivity is reduced. Conversely, if it exceeds 1 × 10 ⁶ W / m ³ , the etching rate becomes too high, and it becomes impossible to control the ratio of the etching rate between the substrate and the photoresist.

The photoresist used in the present invention is not particularly limited, but may be a cyclized rubber-based or novolak-based resin, for example, a cyclized product of polyisoprene and a bisazide compound, or a cresol novolak resin and a naphthoquinonediazide. Compounds and the like can be used.

According to the ridge type fabrication method of the present invention described above, the etching rate ratio between the resist and the optical single crystal substrate or the thin film formed on the substrate is good, and the etching rate of the resist is stable. And on top of that,
Since the uniformity of the etching rate is good, the pattern accuracy can be improved. The reason for this is that CF
_4, unlike the RIE method using active gas CCl ₄ and the like, is considered because etching is performed using a sputtering phenomenon due to an inert gas plasma.

In the present invention, the above pattern precision means a deviation of a ridge width and a ridge height from desired values and indicates a degree of roughness of a side wall of the ridge portion. The greater the roughness of the side wall of the ridge, the lower the propagation loss.

[0019]

An embodiment of the present invention will be described below in more detail with reference to FIG. (Example 1) (1) The LiTaO ₃ single crystal substrate 5 having a thickness of 41.08 mmφ and a thickness of 1 mm (0)
After optically polishing the (001) plane, a LiNbO ₃ single crystal thin film 6 was epitaxially grown to a thickness of 6 μm by a liquid phase epitaxial growth method. This treatment is carried out in the following manner: Na ₂ CO ₃ : 20 mol%,
A mixture of Li ₂ CO ₃ : 30 mol%, V ₂ O ₅ : 40 mol%, Nb ₂ O ₅ : 10 mol%, and MgO added to LiNbO ₃ at 2 mol% is put in a platinum crucible and placed in an epitaxial growth and growth apparatus. Heat to 1100 ° C in an air atmosphere to dissolve the contents of the crucible and cool the resulting melt at a cooling rate of 60 ° C per hour.
The solution was gradually cooled to 939 ° C. The LiTaO ₃ substrate 5 in the melt during obtained as described above, at a rotation (50 rpm) After soaking for 5 minutes while pulling the LiTaO ₃ substrate 5 from the melt, further the melt on After the melt was shaken off by rotating at 1,000 rpm for 30 seconds, it was gradually cooled to a constant temperature.
Thus, LiNb having a thickness of about 6 μm is formed on the LiTaO ₃ substrate 5.
An O ₃ single crystal thin film 6 was produced. The LiTaO ₃ single-crystal substrate 5 and the LiNbO ₃ single-crystal thin film 6 both have an a-axis lattice constant of 5.
At 154 mm, the two were almost the same. This obtained thin film is
Optical polishing was performed to form a thin film having a thickness of 4 μm. This optical polishing
Artificial leather and colloidal silica with an abrasive particle diameter of 500 mm were used for the surface plate.

(2) Next, a photoresist 3 (manufactured by Tokyo Ohka Co., Ltd.) was applied on the thin film 6 (FIG. 5a) produced as described above (FIG. 5b). 1.6 resist thickness
μm. And, using photolithography technology,
A resist pattern 3a having a pattern width of 10 μm was prepared (FIG. 5C). Using the obtained resist pattern 3a as a mask material,
Dry etching was performed using Ar gas plasma. In this dry etching process, the pressure in the chamber before flowing Ar gas was set to 4.03 × 10 ⁻⁴ Pa, the pressure in the chamber after flowing Ar gas was set to 6.06 × 10 ⁻¹ Pa, and R
F power was set to 200W. After the dry etching, the resist pattern 3a remaining on the thin film ridge portion (FIG. 5)
d) was removed with trichlene (FIG. 5e). The step of the ridge portion 1a thus obtained was 1.0 μm. The etching rate ratio between the resist and the LiNbO ₃ 6 was 1: 1. This step was obtained by measuring the etching rate of LiNbO ₃ in advance. (3) The pattern side surface of the ridge waveguide thus manufactured was extremely smooth. The obtained ridge-type optical waveguide was vertically polished, and the propagation loss with respect to a laser having a wavelength of 1.55 μm was measured by a cutback method. The result was 0.8 dB / cm.

Example 2 (1) A ridge-type Ti diffused optical waveguide was manufactured basically by the same ridge-type manufacturing method as in Example 1 according to the method of the present invention.
That is, Ti was thermally diffused after being formed on a LiNbO ₃ substrate by a sputtering method. At this time, the Ti film is 900Å and the diffusion temperature is
The diffusion time was 1000 ° C. and the diffusion time was 10 hours. (2) On the Ti-diffused LiNbO ₃ substrate thus produced,
Photoresist 3 (manufactured by Tokyo Ohka Co., Ltd.) was applied.
The resist thickness is 1.9 μm. Next, a resist pattern 3a having a pattern width of 5 μm is formed using photolithography technology.
Was prepared. Using the obtained resist pattern 3a as a mask material, dry etching was performed using Ar gas plasma.
In this dry etching, the pressure in the chamber before flowing Ar gas was 8.36 × 10 ⁻⁵ Pa, and the pressure in the chamber after flowing Ar gas was 4.83 × 10 ⁻¹ Pa,
The RF power was set to 300W. Thereafter, after the dry etching was completed, the resist pattern 3a remaining on the ridge portion was removed with trichlorene. The step of the ridge portion 1a was 1.5 μm. (3) The pattern side of the ridge waveguide thus obtained was extremely smooth. The obtained ridge-type optical waveguide was vertically polished, and the propagation loss with respect to a laser having a wavelength of 1.55 μm was measured by a cutback method. The result was 1 dB / cm or less.

Example 3 (1) A ridge-type proton exchange optical waveguide was manufactured basically by the same ridge-type manufacturing method as in Example 1 according to the present invention. The difference from Example 1 is that a LiNbO ₃ substrate is immersed in H ₄ P ₂ O ₄ (pyrophosphoric acid), immersion temperature: 210 ° C., immersion time: 1 hour, annealing temperature: 300 ° C., annealing time: 1 hour In that the processing of (2) Photoresist 3 (manufactured by Hoechst Japan) was applied on the proton-exchanged LiNbO ₃ substrate thus produced. The resist thickness is 2.3 μm. Next, using a photolithography technique, a resist pattern 3a having a pattern width of 7 μm was formed. Using the obtained resist pattern 3a as a mask material, dry etching was performed using Ar gas plasma. In this dry etching, the pressure in the chamber before flowing Ar gas was set to 8.92 × 10 ⁻⁴ Pa, the pressure in the chamber after flowing Ar gas was set to 1.06 Pa,
The RF power was set to 400W. After completion of the dry etching, the resist remaining on the ridge portion was removed with trichlorene. The step of the ridge portion 1a was 2.0 μm. (3) The pattern side surface of the ridge waveguide fabricated in this manner was extremely smooth. The obtained ridge-type optical waveguide was vertically polished, and the propagation loss with respect to a laser having a wavelength of 1.55 μm was measured by a cutback method. The result was 1 dB / cm or less.

Example 4 (1) A ridge-type proton exchange optical waveguide was manufactured basically by the same ridge-type manufacturing method as in Example 3 according to the present invention. The difference from the third embodiment is that a LiTaO ₃ substrate 5 is used as the substrate. That is, the LiTaO ₃ substrate was immersed in H ₄ P ₂ O ₄ (pyrophosphoric acid), and immersion temperature: 250 ° C., immersion time: 1.5 hours, annealing temperature: 300 ° C., annealing time: 1 hour were performed. (2) Proton-exchanged LiTaO ₃ substrate 5 thus produced
On top, photoresist 3 (manufactured by Hoechst Japan) was applied. The resist thickness was 2.2 μm. Next, using a photolithography technique, a resist pattern 3a having a pattern width of 7 μm was formed. Using the obtained resist pattern 3a as a mask material, dry etching was performed using Ar gas plasma. At this time, the pressure in the chamber before flowing Ar gas was set to 7.04 × 10 ⁻⁴ Pa, and the pressure in the chamber after flowing Ar gas was set to 9.83 × 10 ⁻¹ Pa. The RF power was 400 W. After the completion of the dry etching, the resist 3 remaining on the ridge portion was removed with trichlorene. The step at the ridge was 2.0 μm. (3) The pattern side surface of the ridge waveguide fabricated in this manner was extremely smooth. The obtained ridge-type optical waveguide was vertically polished, and the propagation loss with respect to a laser having a wavelength of 1.55 μm was measured by a cutback method. The result was 1 dB / cm or less.

(Comparative Example 1) Dry etching by Ar gas plasma using Ti as a mask was performed on LiNbO ₃ on a LiTaO ₃ substrate.
Applied to thin films. That is, a Ti pattern having a pattern width of 10 μm was formed on the thin film obtained in the same manner as in Example 1 by using a lift-off method. Using the obtained Ti pattern as a mask material, dry etching was performed using Ar gas plasma. The pressure in the chamber before flowing Ar gas is 5.08 × 10
⁻⁴ Pa, and the pressure in the chamber after flowing the Ar gas is 6.32 × 10 ⁻¹ Pa. The RF power was set to 200W. After completion of the dry etching, the resist remaining on the ridge portion was removed with trichlorene. Ridge step is 1.0
μm. Considerable roughness was observed on the side surface of the obtained ridge type optical waveguide. The propagation loss of a laser having a wavelength of 1.55 μm, which was perpendicularly polished to the obtained ridge-type optical waveguide by end face polishing, was measured by a cutback method and found to be 3.8 dB / cm. Table 1 shows the pattern accuracy of the ridge waveguides obtained as described above. This pattern precision is defined as L '/ L, where L is the mask opening width and L' is the mask pattern width.

[0025]

[Table 1]

(Comparative Example 2) Dry etching was performed at a pressure in the chamber of 10 ^-2 Pa or more before flowing an inert gas into the chamber. That is, a photoresist (manufactured by Hoechst Japan) was applied on the thin film obtained in the same manner as in Example 1. The resist thickness was 1.7 μm. Next, using photolithography technology, the pattern width
A 10 μm resist pattern was formed. Using the obtained resist pattern as a mask material, dry etching was performed using Ar gas plasma. The degree of vacuum in the chamber before flowing Ar gas was 8.92 × 10 ⁻¹ Pa, and the degree of vacuum in the chamber after flowing Ar gas was 4.38 Pa. Also, RF
Power is 200W. However, the ridge height is 1.0 μm
Before the pattern was obtained, the resist was lost, and the upper part of the waveguide was roughened by etching.

(Comparative Example 3) Dry etching was attempted at a pressure of 7 × 10 ^-2 Pa or less after flowing an inert gas into the chamber. That is, a photoresist (manufactured by Hoechst Japan) was applied on the thin film obtained in the same manner as in Example 1. The resist thickness was 1.7 μm.
Next, using photolithography technology, pattern width 10μ
m resist patterns were produced. Ar gas plasma etching was attempted using the obtained resist pattern as a mask material. The degree of vacuum in the chamber before flowing Ar gas is 6.24
× 10 ^-4 Pa, and the degree of vacuum in the chamber after flowing the Ar gas was 6.82 × 10 ^-2 Pa. The RF power is
200 W. However, no Ar gas plasma was generated,
Etching was not possible.

[0028]

As described above, according to the present invention, since a photoresist is directly used as a mask, a mask pattern can be formed without wasting time. Further, since the etching rate ratio between the resist and the optical single crystal substrate or the thin film on the substrate is good, the resist film may be thin. As a result, a mask pattern with good pattern accuracy can be formed. Also, in the step of performing dry etching, the etching rate of the resist is stable and the etching rate is uniform because of the etching utilizing the sputtering phenomenon by the inert gas plasma. As a result, a ridge type with good pattern accuracy can be manufactured. Therefore, when a ridge-type optical waveguide is manufactured by using the ridge-type manufacturing method according to the present invention, for example, the pattern accuracy of the ridge portion is good, and the roughness of the side wall of the ridge portion can be eliminated. Become.

[Brief description of the drawings]

FIG. 1 is a process chart of a general method for manufacturing a ridge type by a lift-off method.

FIG. 2 is a conventional process diagram for manufacturing a ridge type by ion beam etching using a metal as a mask.

FIG. 3 shows RIE using a photoresist as a mask;
It is a conventional process figure which produces a ridge type by a method.

FIG. 4 is a conventional process diagram for manufacturing a ridge type by ion beam etching using a three-layer structure in which a metal is sandwiched by a photoresist as a mask.

FIG. 5 is a process chart showing a ridge type manufacturing method according to the present invention.

FIG. 6 is an electron micrograph of the optical crystal structure in the optical crystal substrate ridge obtained by the manufacturing method of the present invention.

FIG. 7 is an electron micrograph of an optical crystal structure in an optical crystal substrate ridge obtained by a conventional manufacturing method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Optical single crystal substrate 1a Ridge part 2 Metal 2a Metal pattern 3 Photoresist 3a Resist pattern 4 Optical single crystal thin film 5 LiTaO ₃ substrate 6
LiNbO ₃ single crystal thin film

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-150807 (JP, A) JP-A-4-237032 (JP, A) JP-A-4-247408 (JP, A) 1992 (Heisei 4) Year) Autumn, Proceedings of the 53rd Annual Meeting of the Japan Society of Applied Physics 3rd volume, 939 pages (58) Fields surveyed (Int. Cl. ⁷ , DB name) G02B 6/12-6/14 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. A photoresist is applied on an optical single crystal substrate or a thin film formed on the substrate, and a resist pattern is formed by exposure and development with ultraviolet rays, and an inert gas is formed in a chamber using the resist pattern as a mask. In a method of manufacturing a ridge type by performing dry etching with plasma, the pressure in the chamber before flowing an inert gas into the chamber is set to 10 ⁻² Pa
The optical crystal substrate or a thin film on the optical crystal substrate, wherein the pressure in the chamber after flowing an inert gas into the chamber is set to 7 × 10 ⁻² Pa or more. How to make.