JP2953173B2 - Optical waveguide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide having a large difference in refractive index between a core and a clad.
The present invention relates to an SiONH-based additive containing a refractive index control additive.

[0002]

2. Description of the Related Art With the development of optical fiber communication, optical devices are required to have high productivity, high reliability, no adjustment of coupling, automatic assembly, low loss, and the like. In order to solve the problem, attention has been paid to a waveguide type optical device.

[0003] Among the optical waveguides, a silica glass optical waveguide is particularly promising as a future optical waveguide because of its low loss and very small connection loss with an optical fiber. Conventionally, a flame deposition method shown in FIG. 11 has been known as a method for manufacturing a silica glass optical waveguide. This is because (A) a porous film for a buffer 24a made of quartz glass on a silicon substrate 21 and a porous film for a core made of quartz glass containing a refractive index control additive (Ti or Ge, etc.) on the film. 22B are formed, and (B) a planar optical waveguide film having a buffer layer 24 and a core layer 22b is formed by heating and making them transparent. (C) and (D) A three-dimensional optical waveguide having a convex or rectangular core layer 22 is formed by patterning by a dry etching process using a mask 25. (E) This is realized by forming a clad porous film 23a made of quartz glass on the three-dimensional optical waveguide and (F) forming the clad layer 23 by heating and transparency (Miyashita: Optical waveguide technology). 1. Recent optical waveguide technology, O plus E No.78 pp.59-67).

[0004]

However, as shown in FIG.
The method for manufacturing the silica-based glass optical waveguide of No. 1 has the following problems.

(1) When a core glass film having a high refractive index containing a refractive index control additive is formed on a buffer layer in order to manufacture a glass waveguide having a large refractive index difference between a core and a clad, Since the entire structure is warped due to the difference in the coefficient of thermal expansion, and the amount of warpage is a large value far exceeding 10 μm, it is difficult to pattern an optical circuit with high dimensional accuracy. At this point, there is a limit in increasing the refractive index difference.

(2) Except for the reason (1), it has been found that there is a limit to the difference in the refractive index between the core and the clad. In other words, even when a porous film for a core having a high refractive index is deposited, the additive for controlling the refractive index is volatilized in the sintering process of FIG. Difficult, at most not exceeding 1.47. Therefore, the relative refractive index difference is limited to at most about 1%.

(3) When the core layer containing a large amount of the additive for controlling the refractive index is patterned by a dry etching process as shown in FIG.
Due to the difference in the etching rate between iO ₂ and the above-mentioned additive, the etching side surface is roughened in an uneven shape, which increases scattering loss.

(4) In order to form a transparent and dense film,
Since the sintering process is performed twice (FIGS. 11B and 11F), it takes a long time to manufacture and a utility cost such as an electricity bill, a gas bill, and a water bill, so that it is difficult to reduce the cost.

(5) In the porous film forming process shown in FIGS. 11A and 11E, OH groups are mixed in the film, and the loss of the optical waveguide increases due to absorption loss due to the OH group.

(6) As shown in FIG. 12, the refractive index is 1.4.
When trying to form a high core film of 6 or more, Ti, G
e, Al, P, and other additives for controlling the refractive index must be added in an amount of 10 mol% or more (A). However, the coefficient of thermal expansion changes greatly with that (B). Therefore, the substrate is warped when the core film is formed, which leads to the problem described in (1).

An object of the present invention is to solve the above-mentioned drawbacks of the prior art by finding an optimum core material. Even if the refractive index difference between the core and the clad is increased, the warpage is small.
An optical waveguide type circuit with high dimensional accuracy can be realized at ultra-small size and low cost, and a low-loss optical waveguide with very little OH group contamination.
To provide a wave path .

[0012]

SUMMARY OF THE INVENTION The gist of the present invention is that the outer circumference of a convex core having a refractive index of n _w has a refractive index of n _cp (n _cp <
in the optical waveguide covered with a cladding of n _w), the optical guide
Waveguides, thermal expansion coefficient form a larger substrate than SiO ₂
Made, the core _{is, SiO x N y H z (} x, y> 0, z
> 0 real number)
10 mol% or less of P, Ge, and Al for matching
Or the inclusion of a refractive index control additive such as Ti.
You. Covering the outer periphery of the core with the clad includes the case where the core is partially covered in addition to the entire core. Most of the refractive index of the core is controlled by the content of N, and the content of additives for controlling the refractive index such as Ge and P is slightly suppressed to 10 mol% or less. As a result, an optical waveguide can be realized without significantly changing physical characteristics such as a coefficient of thermal expansion and a softening temperature of a core with respect to a clad or a substrate. Further, by slightly adding a refractive index control additive such as Ge or P to SiO _x N _y H _{z serving} as the core material, the material and the refractive index of the clad layer and the substrate can be freely selected.

[0013]

[Action] The optical waveguide of the present invention, SiO _x N _y H _z in the core
Is used as a main component, so that the high refractive index ratio can be easily controlled by the N content. This SiO _x N _y
The reason why the conventional refractive index control additives such as P and Ge are contained in H _z at 10 mol% or less is mainly to match the thermal expansion coefficient with the cladding and the substrate. That is, when Si or borosilicate glass is used as the substrate, or glass containing additives such as B, P, F, and Ge in SiO ₂ is used, their thermal expansion coefficients are larger than those of SiO _2. Become. For that purpose, the thermal expansion coefficients of the clad and the core need to be close to that.

Since the SiO _x N _y H _z film has a thermal expansion coefficient close to that of SiO ₂ , as described above, a small amount of an additive such as P or Ge is contained so as to approach the substrate. is there. Then, since there is almost no difference in the coefficient of thermal expansion, it is easy to pattern an optical circuit with small warpage and high dimensional accuracy. Further, since the warpage is small, a waveguide type optical circuit having extremely little polarization dependence can be realized. Further, in the SiO _x N _y H _z film, the refractive index of the core can be controlled from 1.458 to nearly 1.60 by adjusting the N content. It is sufficient that the refractive index controlling additive is used not for controlling the refractive index of the core but for controlling the coefficient of thermal expansion in a small amount as described above.

Moreover, the method for manufacturing an optical waveguide of the present invention can form a transparent and dense core film by a plasma CVD method, so that a conventional sintering process is not required and a high refractive index value is maintained. can do. In addition, the conventional refractive index controlling additives such as Ge and P are used in a very small amount as compared with the conventional ones. Therefore, even if patterning is performed by a dry etching process, the etching rate between SiO ₂ and the above-mentioned additives is reduced. The roughness of the etched side surface due to the difference is reduced, and the resulting increase in scattering loss is also suppressed. Furthermore, since the relative refractive index difference can be increased to twice or more (up to about 8%) as compared with the related art,
Various waveguide type optical circuits (for example, Mach-Zehnder type optical filters) can be miniaturized to a small area of one digit or less. As a result, optical circuit loss is significantly reduced, and a large amount of optical circuits can be formed from one wafer substrate, so that significant cost reduction can be expected.

As the substrate, inexpensive borosilicate glass (for example, 7059 glass, 77
40 glass), lead glass, Si, or the like can be used, so that further cost reduction of the optical circuit can be expected. In addition, the sintering process is not required, which reduces manufacturing time and utility costs (electricity, gas,
Water usage fees) can be reduced, and costs can be reduced.

It should be noted that a suitable film can be obtained as the SiO _x N _y H _z film containing Ge, P and the like used in the present invention by forming it by a plasma CVD method. That is, the substrate is heated to 100 to 350 ° C. in a plasma atmosphere,
In this plasma atmosphere, for example, SiH ₄ , Ge
H ₄ , N ₂ O and N ₂ gas are fed to form a film. It is important that the resulting film contains N and H in addition to SiO ₂ , and the refractive index can be controlled by adjusting the N content as described above. That is, the higher the N content, the higher the refractive index can be.

The formed film is SiO _x N _y H _z ,
x decreases as y and z increase. The lower the refractive index,
x approaches 2 and y decreases. Conversely, the higher the refractive index, the more y
Increase, and x becomes smaller than 2. H is low temperature plasma C
It is contained because it is formed by the VD method, and decreases as the refractive index increases, and increases as the refractive index decreases, but the content is in the range of 0.001% by weight to several% by weight. The H content increases as the film is formed in a low-temperature plasma atmosphere, and decreases as the temperature increases. Therefore, the refractive index can be controlled by adjusting the contents of N and H. Therefore, the refractive index of the core including a refractive index controlling additive such as Ge or P suitable for realizing the high refractive index difference, low polarization dependence, low loss, ultra-small, ultra-low cost optical waveguide of the present invention. The rate range is from 1.46 to 1.61. When the refractive index of the core is 1.46 or less, the refractive index is about the same as the refractive index of the clad, and since it does not form a waveguide structure, it cannot be used. Also, undesirably becomes close physical properties to Si _x N _y too contains the N exceeds 1.61.

[0019]

FIG. 7 is a schematic view of a plasma CVD apparatus used in an embodiment of the present invention. This is an apparatus for forming a P _x -containing SiO _x N _y H _z film by a plasma CVD method on a substrate having a refractive index lower than n _w or on a cladding layer formed on the substrate. is there. A shower electrode 6 and a lower electrode 7 serving as two parallel plate electrodes are provided in an upper part and a lower part in the plasma CVD apparatus 5, and a high-frequency voltage is applied between these electrodes 6 and 7 from a high-frequency power supply 12. Then, the inside of the device 5 is evacuated to a vacuum by the exhaust device 14.

The substrate 1 is disposed on the lower electrode 7, and is heated to a temperature of several hundred degrees by applying a voltage 11 to the heater 10 under the electrode 7. The upper shower electrode 6 is provided with a gas (SiH ₄ , P
H ₃ , N ₂ O and, if necessary, N ₂ may be introduced. ) Is uniformly ejected between the parallel plate electrodes, so that it has a shower structure in which the inside is hollow and a large number of holes are formed in the electrode facing surface. The shower electrode 6 is insulated from the plasma CVD device 5 by an insulator 13. With such an apparatus configuration, the SiO _x N _y H _z film containing P is formed under reduced pressure. In FIG. 7, the upper shower electrode 6 and the lower electrode 7 may be mounted opposite to each other, and a film may be formed by blowing gas from below to above.

Here, specific examples of the film forming conditions for realizing the above-described high refractive index will be described. FIG. 8 shows N ₂ O / S
It shows the relationship between the iH ₄ ratio and the refractive index (value at a wavelength of 0.63 μm). In the same figure, the mark ○ indicates the refractive index characteristic when P is not contained, the mark X indicates the refractive index characteristic when P is contained at 4 mol%, and the mark ● indicates the refractive index characteristic when P is contained at 8 mol%. In the case where P is not contained, the refractive index can be changed over a wide range from 1.46 to 1.68, indicating that the relative refractive index difference Δ can be increased to about 8%. However, when the refractive index is 1.
If it exceeds 61, the main material SiO _x N _y H _z
Since the close physical properties to _x N _y, it is not preferable for the optical waveguide of the present invention. Although not become little higher refractive index by slightly containing P, which is the refractive index itself means that determined dominant by SiO _x N _y H _z. That is, P acts to adjust physical properties such as thermal expansion coefficient and softening temperature, and to reduce the H content.

FIG. 9 shows the refractive index characteristics when Ge is contained instead of P. In the case of Ge, the refractive index can be slightly increased by making it slightly contained like P. This Ge also has the effect of reducing the H content in addition to adjusting the thermal expansion coefficient, the softening temperature, and the like, and as a result, low loss can be expected. P, G
When e is contained, an effect is expected that the excitation efficiency can be improved when a laser, amplifier, or sensor is realized by adding a rare earth element such as erbium into the core and exciting with excitation light. it can.

FIGS. 1 to 6 show sectional views of various optical waveguides according to the present embodiment. FIG. 1 shows an optical waveguide structure in which the cladding 3 has a different refractive index. That is, a Si substrate 1, the refractive index to form a first clad 31 of n _cp1, refractive index to form a _{n w (n w> n cp1} ) convex core 2 thereon thereon , The refractive index of the surface of the core 2 is n _cp2 (n _cp2
<N _w ) is a structure covered with the second cladding 32. The core 2 is made of SiO _x N _y H containing P (or Ge).
_z and the first and second claddings 31 and 32 are made of Si.
In order to improve the adhesiveness between the core 2 and the substrate 1 by adjusting the coefficient of thermal expansion between the substrate 1 and the core 2 and adjusting the refractive index (n _cp1 , n _cp2 <n _w ) and further reducing the softening temperature. and it is used (SiO ₂ containing either a or P or B) SiO ₂ containing the B. First clad 31
Is selected from the range of 5 μm to several tens μm, but a thicker film is preferable in terms of reducing loss. The thickness of the second cladding 32 is also selected from the range of several μm to several tens μm, and it is preferable that the thickness is also large in order to reduce the loss. The thickness and width of the core 2 are in the range of several μm to ten and several μm for single mode transmission,
In case of multi-mode transmission, 10 to several tens μm
Is selected from the range.

FIG. 2 shows a borosilicate glass substrate 1
This is an optical waveguide structure aimed at reducing costs. FIG. 3 shows an optical waveguide structure in which not only the refractive index of the first clad 31 and the material of the second clad 32 but also the material are different, although the substrate is a Si substrate. That is, the first clad 31 has no added Si
Using O ₂ and S containing P and B in the second cladding 32
SiO ₂ (or SiO containing either P or B)
₂ ). FIG. 4 shows an optical waveguide structure using a quartz glass (for example, SiO ₂ containing at least one kind of additive for controlling the refractive index such as B, F, P) for the substrate 1. Claddings 31 and 32
Also, an SiO ₂ film containing at least one kind of additive for controlling the refractive index, for example, B, is used.

FIG. 5 shows that the cost is reduced by using borosilicate glass for the substrate 1. The first clad 31 and the core 2 contain B and P, which are additives for refractive index for adjusting the coefficient of thermal expansion, respectively. ing. No SiO added to the second cladding 32
_The reason why ₂ is used is that the second clad 32 is formed by CVD at a low temperature after the pattern of the convex core 2 is formed.
This is because warpage when forming the clad 32 does not pose a particular problem in pattern processing. FIG. 6 shows an embodiment of a ridge type optical waveguide structure having a clad single layer structure in which a quartz glass is used for the substrate 1 and the thickness of the clad 3 is reduced. The clad 3 may be made of SiO ₂ containing at least one additive for controlling the refractive index.
Only ₂ is acceptable. That is, since the clad 3 is coated by the low-temperature CVD method after processing the pattern of the convex core 2, the warpage generated at the time of this coating is small, and even if it occurs, it is a value that does not cause much problem. is there.

The amount of warpage of the substrate is shown in FIG. 3 on the surface of the first clad 31 on the Si substrate 1 having a diameter of 3 inches, and the SiO _x N _y H _z film containing P and Ge shown in FIGS. 8
As a result of measuring the amount of warpage with the formation of μm, the amount of warpage was 5
μm or less. Incidentally The upper limit of the conventional relative refractive index difference delta is less than 1% (see Fig. 8), delta is the conventional warp amount in the range of 10 number μm of 40 number μm in the case of 0.34%, delta Is larger than that, the amount of warpage tends to further increase. As described above, at least one kind of P, Ge, or the like is contained in SiO _x N _y H _z, and by adjusting the content thereof, an optical waveguide having less warpage and a high relative refractive index difference can be realized.

FIG. 10 shows an embodiment of a method for manufacturing an optical waveguide according to the present invention. First, the first cladding film 131 on the substrate 11
The formation is performed (A). The first clad film 131 is formed by a low pressure CV method in addition to the plasma CVD method illustrated in FIG.
It can be performed by a D method, a normal pressure CVD method, a sputtering method, or the like. Next, a core film 12a is formed on the first clad film 131 by the plasma CVD method of FIG. 7 (B). Thereafter, a mask metal film (for example, WSi) 1 is formed on the core film 12a by a sputtering method.
After forming 4, the metal film is patterned by photolithography and dry etching processes (C).

Then, using the metal mask as a mask, the core film 12a is patterned into a convex shape by photolithography and dry etching processes (D). Thereafter, the upper metal film is removed by dry etching (E). Next, the second cladding film 132 is formed by plasma CVD.
The surface of the core 12 formed by the method and having a convex pattern is covered (F). Thereafter, there are two methods as a process. One is a method of skipping (G) and proceeding to (H), and cutting and polishing the end face of the substrate to obtain an optical waveguide device 16, and the other is a method of forming a core and a clad as shown in (G). This is a method of performing a heat treatment in an inert gas atmosphere or a reducing atmosphere at a temperature lower than the softening temperature (800 to 1200 ° C.), and then cutting and polishing.

In the above process, the second clad film 13 is formed.
2 is a plasma CVD method in which a certain film thickness is formed, and then a glass film is added by another film forming method (normal pressure or reduced pressure CVD method, flame deposition method, sprinkling method, etc.). Is also good.

The etching roughness on the side surface of the core was compared with the conventional SiO ₂ glass containing 13 mol% of Ge and the SiO _x N _y H _z glass containing 2 mol% of Ge of the present example. Although the etched side roughness was large irregularities of about 300 to 400 °,
In the case of this embodiment, the unevenness was 200 ° or less. From this, it was found that the present invention can realize an optical waveguide having a lower scattering loss.

The present invention is not limited to the above embodiment. First, metal alkoxide vapors (for example, Si (OCH ₃ ) ₄ , Si (OC ₂ H ₅ ) ₄ , PO
(OC ₂ H ₅ ) ₃ , Ge (OC ₂ H ₅ ) ₄ , B (OC ₂
H _{5) 3,} etc.) may be used. The vapor of the metal alkoxide and the hydride gas may be mixed and used. Further, NH ₃ may be used instead of N ₂ O.

[0032]

As described above, according to the present invention, the following effects are exhibited.

(1) SiO _x N _y H _z whose refractive index can be set to a wide range and a high value is used as a main component of the core, and Ge or P is contained for adjusting a thermal expansion coefficient with a clad or a substrate. As a result, a high-refractive index difference, low polarization dependence, low loss, and ultra-miniaturized optical waveguide can be realized for various substrates.

(2) G used for adjusting the coefficient of thermal expansion
Since the contents of e, P and the like are as small as 10 mol% or less, it is possible to minimize the etching roughness when etching the convex pattern of the core.

(3) A low-cost substrate can be used, and further cost reduction can be realized. Also,
Since the substrate can be selected according to the purpose of use and use, a more general-purpose optical waveguide circuit can be realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an optical waveguide using Si for a substrate and having different cladding refractive indexes according to an embodiment of the present invention.

FIG. 2 is a sectional view of an optical waveguide using borosilicate glass for a substrate according to an embodiment of the present invention.

FIG. 3 is a cross-sectional view of an optical waveguide using Si for a substrate and having different cladding refractive indices and materials according to an embodiment of the present invention.

FIG. 4 is a cross-sectional view of an optical waveguide using quartz-based glass for a substrate according to an embodiment of the present invention.

FIG. 5 is a cross-sectional view of an optical waveguide that uses borosilicate glass for a substrate according to an embodiment of the present invention and has different cladding refractive indexes and materials.

FIG. 6 is a cross-sectional view of a ridge-type optical waveguide using quartz glass for a substrate according to an embodiment of the present invention.

FIG. 7 is a schematic view showing an embodiment of a plasma CVD apparatus for forming a core film according to the present invention.

FIG. 8 is a refractive index characteristic diagram of a P-doped core film according to an embodiment of the present invention.

FIG. 9 is a refractive index characteristic diagram of a Ge-doped core film according to an embodiment of the present invention.

FIG. 10 is a schematic process diagram showing a method for manufacturing an optical waveguide according to an embodiment of the present invention.

FIG. 11 is a schematic process diagram showing a conventional method for manufacturing an optical waveguide.

FIG. 12 shows a conventional S containing various refractive index controlling additives.
refractive index characteristic diagram and thermal expansion coefficient characteristic diagram of iO _2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Si substrate 2 Core 3 Cladding 31 First cladding 32 Second cladding

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02B 6/12

Claims (1)

(57) [Claims] (1) the refractive index is n_wThe outer circumference of the convex core of
n_cp(N_cp<N_w) Optical waveguide covered with cladding
hand,The optical waveguide has a thermal expansion coefficient of SiO. _Two Larger than
And the core is made of SiO 2 _x N _y H
_z (Real number of x, y> 0, z> 0)
Of 10 mol% or less so as to match the coefficient of thermal expansion with
Or contains an additive for controlling the refractive index such as Ge.thing
An optical waveguide characterized by the above.