JP6280022B2 - Optical waveguide fabrication method - Google Patents

Description

The present invention relates to an optical waveguide manufacturing method for manufacturing an optical waveguide composed of SiO _x .

  An optical waveguide using quartz has a low propagation loss in a communication wavelength band, and is widely used in communication optical circuits. By the way, among optical circuits for communication, a circuit using the phase of light, for example, an arrayed-waveguide grating type optical multiplexer / demultiplexer (AWG) or a delay interferometer, has an effective refractive index of the optical waveguide. Depends on. Therefore, a shift in the effective refractive index due to a minute change in the shape of the optical waveguide in manufacturing produces a phase error and greatly affects the element characteristics. For this reason, it is necessary to correct the error after the optical waveguide is manufactured.

  In general, the effective refractive index correction is performed by finely adjusting the refractive index of the core or the clad of the formed optical waveguide after the optical waveguide is manufactured. In quartz optical waveguide elements that have been commercialized, Ge-doped silica takes advantage of the fact that the refractive index increases when it absorbs ultraviolet rays. The error of the rate is corrected (see Patent Document 1).

  In an optical waveguide using silica doped with Ge having such characteristics, a process for forming a high-quality film is important. In a process that is currently commercialized, a high-quality silica film doped with Ge is formed by a high-temperature process (> 1000 ° C.) called a FHD (Flame Hydrolysis Deposition) method, and this film is used as a core.

On the other hand, in recent years, attention has been focused on quartz-based (SiO _x ) optical waveguides manufactured at a low temperature of 400 ° C. or lower using plasma CVD (Chemical Vapor Deposition) instead of high-temperature processes such as FHD. By using a low-temperature process of 400 ° C. or lower, thermal damage to active elements using semiconductors such as silicon and germanium can be prevented, and high-performance quartz passive elements and high-speed / small semiconductor active elements can be integrated (non-integrated) Patent Document 1).

Japanese Patent Application Laid-Open No. 06-051145

T. Tsuchizawa, K. Yamada, T. Watanabe, S. Park, H. Nishi, R. Kou, H. Shinojima, and S. Itabashi, "Monolithic Integration of Silicon-, Germanium-, and Silica-Based Optical Devices for Telecommunications Applications ", IEEE Journal of Selected Topics in Quantum Electronics, vol.17, no.3, pp.516-525, 2011.

However, SiO _x does not have the property of changing the refractive index in response to light, like silica doped with Ge. For this reason, when SiO _x is used, there is a problem that the effective refractive index cannot be corrected by the ultraviolet irradiation amount after the optical waveguide is formed as described above.

The present invention has been made to solve the above-described problems, and an object thereof is to make it possible to correct the effective refractive index of an optical waveguide using SiO _x after the optical waveguide is formed. .

An optical waveguide manufacturing method according to the present invention includes an optical waveguide forming step of forming an optical waveguide using a SiO _x film formed under a deposition condition with a film forming temperature of 400 ° C. at the maximum, and a maximum of 400 optical waveguides formed. And an adjusting step of adjusting the effective refractive index of the optical waveguide by heating at a temperature of ° C.

In the optical waveguide manufacturing method, the heating temperature may be 350 ° C. or lower in the adjusting step. In the optical waveguide forming step, it is better to set the film forming temperature to 150 ° C. or lower. In the optical waveguide forming step, the SiO _x film may be formed by PE-CVD.

  In the optical waveguide manufacturing method, the optical waveguide constitutes an array optical waveguide diffraction grating type optical multiplexer / demultiplexer, and in the adjusting step, the effective refractive index of the array optical waveguide diffraction grating type optical multiplexer / demultiplexer is adjusted.

As described above, according to the present invention, it is possible to obtain an excellent effect that the effective refractive index of the optical waveguide using SiO _x can be corrected after the optical waveguide is formed.

FIG. 1 is a flowchart for explaining an optical waveguide manufacturing method according to an embodiment of the present invention. FIG. 2 is a configuration diagram showing a configuration of an array optical waveguide diffraction grating type optical multiplexer / demultiplexer (AWG) configured by an optical waveguide using SiO _x . FIG. 3 is an explanatory diagram for explaining the relationship between the center wavelength error Δλ with respect to the center wavelength design value λ ₀ and the processing error ΔW of the cross-sectional dimensions of the core constituting the optical waveguide. FIG. 4 is an explanatory diagram illustrating the concept of center wavelength adjustment on the spectrum. FIG. 5 is a characteristic diagram showing experimental results showing the relationship between the refractive index of each of the core film (a) and the clad film (b) and the heating temperature. Figure 6 is a characteristic diagram showing a core diameter machining error ΔW is -0.2 m, 0.0 m, the relationship between the heat treatment temperature and n _e (T) in the case of + 0.2 m. FIG. 7 is a characteristic diagram showing the relationship between the AWG center wavelength error Δλ and the heating temperature when the core diameter processing error ΔW is +0.2 m, 0 m, and −0.2 μm.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a flowchart for explaining an optical waveguide manufacturing method according to an embodiment of the present invention.

First, in step S101, an optical waveguide is formed using a SiO _x film formed under deposition conditions where the film forming temperature is at most 400 ° C. (optical waveguide forming process). For example, an SiO _x film having a refractive index of 1.470 as a design value is formed on the substrate to form a lower cladding layer. Next, an SiO _x film having a refractive index of 1.510 as a design value is formed on the lower clad layer, and the core is formed by patterning the SiO _x film by a known lithography technique and etching technique.

Next, an SiO _x film having a refractive index of 1.470 as a design value is formed on the lower cladding layer and the core to form an upper cladding layer. For example, each SiO _x film may be formed by PE (Plasma Enhanced) -CVD (Chemical Vapor Deposition) at a film formation temperature of 150 ° C. By the process described above, an optical waveguide using SiO _x can be obtained.

  Next, in step S102, the effective refractive index of the optical waveguide is adjusted by heating the optical waveguide formed in step S101 at a maximum temperature of 400 ° C. (adjustment process). Note that more preferably, the temperature condition of the heat treatment described above is 350 ° C. or lower.

As described above, the optical waveguide formed using SiO _x can be corrected after the effective refractive index deviation (error) from a desired value by heating after forming the optical waveguide. It is known that the refractive index of the SiO _x film is reduced by the heat treatment after the film formation. This is presumably because the bonding state in the film is stabilized by heat treatment. The inventors have found that this phenomenon is manifested not only in the case of a single film but also in a state where the film is patterned and other films having different compositions are laminated to form an optical waveguide, and the present invention has been achieved.

Even after the optical waveguide is formed using SiO _x , the refractive index is reduced by the heat treatment, and the adjustment amount of the refractive index can be controlled by the temperature condition of the heat treatment. As a result, it is possible to adjust the deviation of the effective refractive index due to a minute change in the shape of the optical waveguide in manufacturing the optical waveguide using SiO _x .

Hereinafter, the optical waveguide using SiO _x will be described in more detail by taking an array optical waveguide diffraction grating type optical multiplexer / demultiplexer (AWG) as an example. Hereinafter, adjustment of the center wavelength of the AWG will be described. As shown in FIG. 2, the AWG includes an input optical waveguide 201, an input slab optical waveguide 202, an array optical waveguide 203, an output slab optical waveguide 204, and an output optical waveguide 205.

  The wavelength-multiplexed incident light incident on the input optical waveguide 201 is spread by diffraction in the input slab optical waveguide 202 and is incident on the array optical waveguide 203. The array optical waveguide 203 has an optical path length difference ΔL between adjacent optical waveguides (cores). For this reason, the incident light propagated through the array optical waveguide 203 is diffracted so as to form an equiphase surface in the output slab optical waveguide 204. Since the imaging position of the wavefront of this diffracted light differs depending on the wavelength, it is condensed on each output channel of the output optical waveguide 205 for each wavelength and demultiplexed.

Here, if the wavelength at which the diffraction angle θ in the output slab optical waveguide 204 is 0 ° is the center wavelength λ ₀ , the center wavelength λ ₀ is expressed by the following equation (1).

Here, _ne is the effective refractive index of the optical waveguide, and m is the diffraction order. When actually manufacturing the AWG, affect the effective refractive index n _e of the optical waveguide variation of minute optical waveguide shape, the center wavelength of the AWG after fabricated deviates from the design value. FIG. 3A shows a calculation example of the relationship between the shift amount (center wavelength error) Δλ with respect to the center wavelength design value λ ₀ and the processing error ΔW of the cross-sectional dimensions of the core constituting the optical waveguide.

As shown in FIG. 3B, the target optical waveguide is composed of a lower clad layer 302 formed on a substrate 301 and a core 303 formed thereon. The core 303 is designed to have a width of 3 μm and a thickness of 3 μm in cross-sectional view. The lower cladding layer 302 has a refractive index of 1.470, and the core 303 has a refractive index of 1.510. Further, the array optical waveguide 203 by the optical waveguide having this configuration has a center wavelength λ ₀ = 1550 nm as a design value. Further, the core width machining error is ΔW shown in FIG.

  As shown in FIG. 3A, it can be seen that the center wavelength shifts to the short wave side as the core width decreases, and the center wavelength shifts to the long wave side as the core width increases.

The center wavelength shift due to the processing error of the core width described above can be corrected by fine adjustment of the refractive index of the core and the clad by heat treatment after the optical waveguide is manufactured. As described above, in the SiO _x film, the bonding state in the film is stabilized by the heat treatment after the film formation, and the refractive index is decreased. Further, when the heating temperature is increased, the refractive index reduction amount is also increased. Moreover, these changes are manifested even after being formed in the optical waveguide. Therefore, by adjusting the heat treatment temperature after forming the optical waveguide, the center wavelength of the AWG after fabrication can be matched with the design center wavelength value.

FIG. 4 is an explanatory diagram illustrating the concept of center wavelength adjustment on the spectrum. The refractive index of the core and clad is made higher than the design value, and the produced center wavelength (λ _as-depo ) before heat treatment is sufficiently longer than the design center wavelength value (λ ₀ ). Keep it. After the fabrication, the center wavelength of the AWG is measured, the temperature required to match the design center wavelength value is estimated, and heat treatment is performed at this temperature. By this adjustment, even if there is a processing error in the production of the optical waveguide, the center wavelength can be adjusted to the design value.

In the manufacturing method described above, the refractive index of the SiO _x film (core film) serving as the core is such that the central wavelength λ _as-depo immediately after the AWG is manufactured (before heating) is longer than the design value λ _0. In addition, it is important to form (manufacture) the SiO _x film (cladding film) used as the cladding with a refractive index higher than the design value. In addition to this, it is important that λ _as-depo is within a range that can be corrected to the design value by heat treatment.

  In other words, the range of the center wavelength that can be adjusted is limited to the following two conditions. Therefore, if each film is formed with a refractive index that is higher than the design value to produce an AWG, it may not be possible to shift to the design value center wavelength by heat treatment.

[Condition 1] Restriction for preventing deterioration of characteristics of semiconductor active element An optical waveguide composed of SiO _x can be manufactured by a low-temperature process, and does not deteriorate other monolithically formed semiconductor elements. The heat treatment after the production of the optical waveguide is also a low temperature process. In general, in a CMOS compatible process, the temperature needs to be 400 ° C. or lower, and more preferably, a temperature range of 350 ° C. or lower.

[Condition 2] Limitation by lower limit of refractive index value In a relatively low temperature range, the SiO _x film has a refractive index that decreases as the heating temperature rises, but on the high temperature side, the refractive index tends to increase as the temperature increases. Often shown. Table 1 below shows an example (experimental value) of the relationship between the refractive index of the SiO _x film and the heat treatment temperature. Here, a SiO _x film was deposited under a temperature condition of 150 ° C. by an ECRPE (Plasma Enhanced) -CVD method using plasma by Electro Cyclotron Resonance (ECR). “As-depo” is a state before film formation and before heat treatment. In this case, as shown in Table 1, since the lower limit value (1.5018, 350 ° C.) exists for the refractive index value, the correctable range is limited to the range shown in Table 1. Note that the lower limit value of the refractive index and the temperature reaching the lower limit value are not limited to those shown in Table 1 because they depend on the film forming method and the composition. The tunable range of the SiOx film used for AWG fabrication needs to be grasped beforehand from the data shown in Table 1.

[Example]
Hereinafter, it demonstrates in detail using an Example. Hereinafter, a case will be described in which the center wavelength shift due to the maximum AWG processing error ΔW = ± 0.2 m due to SiO _x described with reference to FIG. 3 is adjusted.

As described above, the design value of the AWG is set such that the core diameter is 3 μm square, the core refractive index is 1.510, the cladding refractive index is 1.470, and the center wavelength λ ₀ = 1550 nm. In this example, the refractive index of the core film at the time of fabrication was set to 1,516, and the refractive index of the cladding film was set to 1.473, so that an optical waveguide was fabricated. Each SiO _x film is formed at a substrate temperature condition of 150 ° C. by ECRPE-CVD using ECR plasma. Since ECRPE-CVD has a high gas decomposition rate and can control a wide range of refractive index at low temperature, it is suitable for manufacturing an SiO _x optical waveguide. Even in general PE-CVD, an SiO _x film can be formed (deposited) under a substrate temperature condition lower than 350 ° C.

A state in which the refractive index of the core film and the refractive index of the cladding film change with the heating temperature will be described with reference to FIG. FIG. 5 is a characteristic diagram showing experimental results showing the relationship between the refractive index of each of the core film (a) and the clad film (b) and the heating temperature. From the results of FIG. 5, the relationship between the refractive index value n _core of the core film and the heat treatment temperature T and the relationship between the refractive index value n _clad of the cladding film and the heat treatment temperature T were approximated by the following equations.

Core film: n _core = −3.1241 × 10 −5 × T + 1.516 (2)
Clad film: n _clad = −1.7239 × 10 −5 × T + 1.473 (3)

Next, (2), (3) from equation was calculated the relationship between the effective refractive index value n _e (T) and the heat treatment temperature in the core diameter machining error ΔW = -0.2μm~ + 0.2μm. The core diameter machining error [Delta] W, shown -0.2 m, 0.0 m, the relationship between n _e (T) and the heat treatment temperature in + 0.2 m (calculated results) in Fig. In a temperature range of 350 ° C. or lower that does not affect other monolithically formed elements, the effective refractive index n _e (T) changes within the range shown in FIG. 6 as the heat treatment temperature increases.

Finally, to calculate the relation between the center wavelength error Δλ and n _e (T) at each annealing temperature obtained from 6 by the following equation (4). In the expression, n _e0 is an effective refractive index value in the design value optical waveguide.

  FIG. 7 shows the results of calculating the relationship between the AWG center wavelength error Δλ and the heating temperature when the core diameter processing error ΔW is +0.2 m, 0 m, and −0.2 μm. After fabrication (heat treatment at 0 ° C.), the center wavelength is sufficiently shifted to the long wavelength side in any processing error. Further, it can be confirmed that the center wavelength error Δλ is reduced by the heat treatment (the center wavelength is shifted to the short wave side). At the heating temperatures indicated by three circles in FIG. 7, the center wavelength error Δλ in each processing error becomes 0, and the center wavelength is adjusted to the design value.

  The heating temperature range required for center wavelength adjustment at ΔW = −0.2 to +0.2 μm is 133 ° C. to 255 ° C. This is a general back-end process temperature range (<350 ° C.) in LSI, and can be used as a monolithic integration process with semiconductor active devices such as MOS transistors.

As described above, in order to satisfy the heating temperature range of 350 ° C. or lower, there is an upper limit to the refractive index of the SiO _x film during film formation. Table 2 below requires adjustment of the center wavelength shift by ΔW = −0.2 μm to +0.2 μm when the refractive index of SiO _x at the time of film formation is 1.516, 1.518, and 1.520. Indicates the heating temperature range. It can be seen that when the refractive index during SiO _x film formation is 1.52, the temperature range required to shift the center wavelength to the design value exceeds 350 ° C., and therefore wavelength adjustment cannot be performed.

  From the above, it can be seen that an AWG having a desired center wavelength may be manufactured by the following procedure.

[First step]
A plurality of optical waveguides are manufactured by using a plurality of core films and cladding films each having a different refractive index, and in each optical waveguide, a refractive index adjustable range (n1 ≦ n ≦ n2) by heat treatment is experimentally obtained. Note that n2 is the refractive index before the heat treatment, and n1 is the refractive index after the heat treatment at the maximum temperature.

[Second step]
A processing error range W1 ≦ ΔW ≦ W2 of the core shape is experimentally obtained.

[Third step]
When the processing error is W1, the range of the central wavelength error Δλ of the AWG after the heat treatment (n1 ≦ n ≦ n2) is obtained for each optical waveguide.

[Fourth step]
In the case of processing error W2, in each optical waveguide, the range of the central wavelength error Δλ of the AWG after the heat treatment (n1 ≦ n ≦ n2) is obtained.

[Fifth step]
In either case of W1 or W2, an AWG is manufactured using an optical waveguide having a center wavelength error Δλ = 0.

As described above, according to the present invention, after forming the optical waveguide by using a SiO _x film, since to adjust the effective refractive index by heating the optical waveguide, optical with SiO _x The effective refractive index of the waveguide can be corrected after the optical waveguide is formed.

The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, the deposition of the SiO _x film is not limited to the PE-CVD method, and the SiO _x film may be deposited by a sputtering method.

  DESCRIPTION OF SYMBOLS 201 ... Input optical waveguide, 202 ... Input slab optical waveguide, 203 ... Array optical waveguide, 204 ... Output slab optical waveguide, 205 ... Output optical waveguide, 301 ... Substrate, 302 ... Lower clad layer, 303 ... Core.

Claims (5)

An optical waveguide forming step of forming an optical waveguide using a SiO _x film formed under deposition conditions with a film forming temperature of at most 400 ° C .;
An adjusting step of adjusting the effective refractive index of the optical waveguide by heating the formed optical waveguide at 400 ° C. at the maximum. In the optical waveguide manufacturing method according to claim 1,
In the adjusting step, the heating temperature is set to 350 ° C. or lower. In the optical waveguide manufacturing method according to claim 1 or 2,
In the optical waveguide forming step, the film forming temperature is set to 150 ° C. or lower. In the optical waveguide manufacturing method according to any one of claims 1 to 3,
In the optical waveguide forming step, the SiO _x film is formed by a PE-CVD method. In the optical waveguide manufacturing method according to any one of claims 1 to 4,
The optical waveguide constitutes an arrayed waveguide diffraction grating type optical multiplexer / demultiplexer,
In the adjusting step, an effective refractive index of the arrayed waveguide grating optical multiplexer / demultiplexer is adjusted.