JPH02163706A - Silicon light waveguide and production thereof - Google Patents

Abstract

PURPOSE:To obtain the silicon light waveguide which is low in light propagation loss and is easy to handle by embedding a silicon core part by a quartz glass clad layer contg. a prescribed dopant. CONSTITUTION:A silicon layer 2 having the free carrier concn. lower than the free carrier concn. of a silicon substrate 1 is first epitaxially grown on the substrate 1. The unnecessary part of the silicon layer having the low free carrier concn. in then removed to form the ridge-shaped silicon core part 2a. The quartz glass clad layer 3b contg. the dopant to lower the softening temp. of the quartz glass is formed so as to embed at least the ridge-shaped silicon core part 2a. The influence of the fluctuation in the boundary with the quartz glass clad layer is drastically decreased by adopting the constitution of the silicon light guide provided with such thick quartz glass clad layer. The silicon light guide which is low in loss and is easy to handle is thus obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a silicon guided waveguide used in the field of optical integrated circuits and a method for manufacturing the same. The present invention relates to a simple silicon guided waveguide and a method for manufacturing the same.

(Left below) [Prior Art] There is an increasing demand for practical optical integrated circuits in the fields of optical communication, optical information processing, etc. In order to construct an optical integrated circuit, it is fundamental to form a low-loss leading waveguide on a flat substrate. Conventionally, various guiding waveguide structures and their manufacturing methods have been proposed using materials such as glass, plastic, phone crystal, and compound semiconductors.

If it were possible to form optical waveguides using silicon material on a crystalline silicon substrate (silicon wafer), which has established itself as a basic material in the LSI technology field today, it would be possible to economically provide optical integrated circuits. Not only LS
The emergence of new functional circuits through fusion with I is also expected.

In fact, from the above-mentioned viewpoint, silicon guided waveguides have been proposed.

FIGS. 5(a) and 5(b) are cross-sectional views showing examples of cross-sectional structures of conventionally proposed silicon guided waveguides. The illustrated structure is based on the academic journal IEEE Journal of Qu.
antum Electronics, Vol, QE-
22, No, 6. (1986) pp.

873-879 and Appl, Phys, LeLt,
, Vol. 51. No. 1 (1987) pp. 6-8
has been disclosed. FIG. 5(a) shows a simple ridge-type silicon optical waveguide, and FIG. 5(b) shows a thin film 5j (h
A cladded ridge-type silicon guided waveguide is shown.

Here, 1 is a silicon substrate containing a large amount of dopants and having a high free carrier concentration, and also functions as a lower cladding layer. 2 is a silicon layer with a low free carrier concentration formed on the silicon substrate 1 by an epitaxial growth method. 2a is a silicon leading waveguide core portion formed by processing a part of the silicon layer 2 into a ridge shape. Reference numeral 3 denotes a cladding layer of Sin in the form of a thin film disposed on the silicon layer 2 so as to cover the core portion 2 .

For the substrate 1, for example, the dopant P is 2×10"cm
Using an n1 type silicon substrate containing about -' silicon layer 2
The dopant P is 7.5 Xl01′coi”
The dimensions of the core portion 2a are approximately 10 μm in width, approximately 4 μm in height at the ridge portion, and approximately 7 μm in overall height.
Thin @sio in b), the thickness of the cladding layer 3 is at most 0
．． It is about 5μ melon or less.

In FIG. 5(a) or (b), there is a gap between the core part 2a and the substrate 1 of Δn=1 due to the difference in free carrier concentration.
．． A refractive index difference of about 5×10 −3 occurs, and in combination with the lateral light confinement effect due to the ridge shape, the core portion 2a can perform a desired light propagation effect.

[Problems to be Solved by the Invention] However, the optical propagation loss of these conventional silicon guided waveguides is extremely large, approximately 15 to 20 dB/cm in the wavelength band of 1 and 3 μm, and is difficult to solve in the design, manufacture, and application of optical integrated circuits. This was a serious constraint. The reason why the light propagation loss is so large is presumed to be that there is processing roughness on the ridge side surface of the core part 2a in FIG. 5(a), which causes light scattering loss at the interface between the core part and the surrounding air. Ru. In the structure of FIG. 5(b), although the core portion 2a is covered with the thin IIjsi02 cladding layer 3, the interface fluctuation between the thin gsiO□ cladding layer 3 and the core portion 2a still exists.

Moreover, since the thickness of the thin film Sin and the cladding layer 3 is insufficient, the signal light propagating through the core portion 2a is transmitted through the thin film 5i02.
Even the fluctuation of the surface of the cladding layer 3 was felt, and it is presumed that this also caused an increase in light scattering loss.

The optical propagation loss borderline for whether or not an optical waveguide can actually be used as a component of an optical integrated circuit is 2 to 3 dB/cm.
Therefore, the conventional silicon guided waveguide shown in FIG. 5(a) or (b) is not suitable for practical use.
The emergence of a low-loss silicon guided waveguide structure and its manufacturing method has been desired.

Furthermore, in the conventional silicon guiding waveguide structure shown in FIGS. 5(a) and 5(b), the core portion 2a is exposed near the surface or partially exposed, so light propagation is affected by adhesion of external particles. There were also mounting problems, such as the loss being more likely to increase and the core part being more likely to be damaged.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks and provide a silicon guided waveguide that has low optical propagation loss and is easy to handle, and a method for manufacturing the same.

[Means for Solving the Problems] In order to achieve such an object, in the present invention, a thick silica-based glass cladding layer containing, for example, B2O3 or P2O5 as a dopant that lowers the softening temperature of silica-based glass is used. Completely covering the wave core part,
The core part is buried.

That is, the silicon guided waveguide of the present invention includes a silicon substrate that also serves as a lower cladding layer, and is arranged on the silicon substrate,
It is characterized by comprising a silicon core portion having a lower free carrier concentration than a silicon substrate, and a silica-based glass cladding layer disposed to embed the silicon core portion and containing a dopant that lowers the softening temperature of silica-based glass. .

Here, a diffusion layer having a free carrier concentration higher than that of the silicon core may be provided on the surface of the silicon core that is in contact with the silicon substrate and the surface that is in contact with the silica-based glass ground layer.

In manufacturing such a silicon guided waveguide, the manufacturing method of the present invention includes a step of epitaxially growing a silicon layer with a higher free carrier concentration than the silicon substrate on a silicon substrate, and removing an unnecessary portion of the silicon layer with a low free carrier concentration. forming a ridge-shaped silicon core, and forming a silica-based glass cladding layer containing a dopant that lowers the softening temperature of the silica-based glass by embedding at least the ridge-shaped silicon core. It is characterized by:

Here, the quartz-based glass crat layer is formed by depositing a glass fine particle layer for forming a cladding layer so as to embed the core part by a flame hydrolysis reaction of glass forming raw material gas, and then heating this glass fine particle layer together with the substrate. It can be formed by transparent vitrification.

[Function] The silicon guided waveguide according to the present invention is completely shielded from the outside air and is completely free from the influence of interfacial fluctuations between the cladding layer and the air layer. In addition, in the step of heating a glass fine particle layer deposited on a silicon substrate to form a transparent glass for forming a cladding layer, B20. Alternatively, a dopant such as p2o diffuses into the core surface and increases the free xeria concentration on the core surface, thereby forming a substantial silicon crat layer on the core surface. As a result, the influence of interfacial fluctuation with the silica-based glass cladding layer can be drastically reduced. As described above, according to the present invention, a silicon guided waveguide with low loss and easy to handle can be obtained.The silicon guided waveguide according to the present invention is completely embedded with a silica-based glass cladding layer with a thickness of several 10 μm. This is significantly different from conventional silicon guided waveguides.

E Example] Hereinafter, examples of the present invention will be described in detail with reference to the drawings.

FIGS. 1(a), (b), (c), and (d) are cross-sectional views for explaining sequential steps in an embodiment of the method for manufacturing a silicon guided waveguide according to the present invention. The steps will be explained step by step below.

As shown in the rfSI diagram (a), first, a silicon layer 2 (here n A mold) is epitaxially grown to a thickness of about 7 μm.

Next, by a photolithography process using a photoresist and a microfabrication method using reactive ion etching, a desired portion of the silicon layer 2 is left in a ridge shape (about 4 μm in height), as shown in FIG. 1(c). As shown in FIG. 2, a silicon core portion 2a is formed.

Subsequently, as shown in FIG. 1(c), the ridge-shaped core portion 2
A glass fine particle layer 3a for forming a quartz-based cladding layer is deposited on the silicon layer 2 so as to cover the layer a. Here, the glass fine particle layer 3a is deposited using 5iC14 and PCJ! This is carried out using a flame hydrolysis reaction of a glass-forming raw material gas mixed with 3. That is, the raw material gas is supplied into an oxyhydrogen torch, and the glass particles generated in the flame are transferred to the core part 2.
It is sprayed so as to cover the silicon layer 2 containing a.

The glass fine particle layer 3a deposited in this way is heated to about 1150°C together with the substrate 1 in an electric furnace to sinter the glass fine particle layer 3a and make it transparent vitrified.
), a silica-based glass cladding layer 3b (having a thickness of about 25 μm on the ridge-shaped core portion 2a) containing P;05 as a dopant is formed.

When the optical propagation loss (wavelength: 1.3 μm) of the silicon guiding waveguide produced by the above process was actually measured, it was about 2 dB/cm. The reason for the low loss of the silicon guided waveguide according to the present invention, which has a loss value that is more than one order of magnitude smaller than the optical propagation loss of the conventional silicon guided waveguide, will be explained next.

FIG. 2 is a cross-sectional view showing a detailed cross-section of the silicon guiding waveguide of the present invention manufactured through the above steps.

In the transparent vitrification process described above, the P component of the dopant (hos here) contained in the silica-based glass cladding layer 3b is applied to the upper surface of the silicon layer 2 including the silicon optical waveguide core part 2a. There is an upper diffusion layer 4 in which the free carrier concentration increases by diffusion into the substrate 2a and becomes n" type. In this transparent vitrification process, the dopant (in this case P) contained in the substrate 1 is also diffused into the substrate 1. A lower diffusion layer 5 is formed by being slightly diffused on the lower surface of the silicon layer 2.

Here, in particular, the core part 2a and the quartz-based glass ground layer 3
It is characteristic that the boundary with b is not steep, but is gentle due to the above-mentioned diffusion phenomenon. As a result, the interface fluctuation existing at the boundary between the core part 2a and the cladding layer 3b becomes smaller in appearance, and the occurrence of light scattering loss, which has been a problem in the past, is suppressed, and the advantage of low light propagation loss in the present invention is realized. It is thought that this will be brought about. Of course, in the present invention, the cladding layer is much thicker than the conventional structure shown in FIG. This is an important factor.

In the embodiment described above, the diffusion of the P component contained as a dopant in the cladding layer 3b contributes to the formation of the fine structure of the core portion 2a.
plays an important role. This amount of diffusion is
It is greatly influenced by the temperature at which b is made into transparent glass. That is, when the transparent vitrification temperature is about 1000°C or lower, diffusion hardly occurs and the effect of reducing light propagation loss is low.6 Conversely, when the transparent vitrification temperature is about 1300°C
If the temperature is above ℃, the diffusion depth is too deep and the core part 2
The low free carrier concentration region of a disappeared, and the light propagation effect itself was lost.

The minimum temperature required for transparent vitrification of the silica-based glass fine particle layer 3a is approximately determined by the amount of dopant added.
It is necessary to make the cladding layer 3b contain about mol% or more. Such setting of the glass composition can be controlled by the composition of the glass forming raw material gas. When the added concentration of P2O exceeds about 20 mol%, cladding FJ3b
In addition, the thermal expansion coefficient of the cladding layer 3b becomes larger than that of the silicon substrate 1, which makes the cladding layer 3b more likely to crack, which is not desirable in practice.

As mentioned above, the silicon guided waveguide of the present invention has a core portion 2.
The basic structure is that a is embedded in a thick quartz-based cladding layer 3b, and the appropriate thickness range of this cladding layer 3b is approximately 5 to 100 μl. If the film thickness is less than about 5 μl, not only the loss reduction effect will be weakened, but also
This is undesirable because it is easily damaged during handling. The film thickness is
If it exceeds about 100 μm, not only is the material wasted, but the silicon substrate 1 also warps, which is also undesirable in terms of handling.

As means for depositing the silica-based glass fine particle layer 3a on the silicon substrate 1, various methods are known in addition to the method using the flame hydrolysis reaction described above.
The above method is optimal for depositing a so-called film thickness in the 0 μm range with high quality.

FIG. 3 is a sectional view showing a cross section of a silicon guiding waveguide provided with a current injection part as a second embodiment of the present invention. The difference between this example and Example 1 is that B20, instead of P2O, was used as a dopant in the silica-based glass cladding layer 3b.
．． A part of the cladding layer 3b is provided with an opening 6 that reaches the ridge-shaped core portion 2a, and an electrode 7 is provided on the core portion 2a through the opening 6. 1 is that a lower electrode 8 is disposed on the lower main surface.

In forming the structure shown in FIG. 3, first, through the same steps as in Example 1, a silica-based glass cladding JiJ3b was deposited and made transparent so as to bury the ridge-shaped core portion 2a. However, as the glass forming raw material gas, S
i C11a -P C,Q Instead of 3 system, 5iCj
! The composition of the cladding layer 3b was set to be 5j02-8203 based using 4-BCIl,s based. Next, a part of the cladding layer 3b above the core part 2a is formed with a width of 6 μm reaching the core part.
An opening 6 having a length III+ was formed by reactive ion etching, and then an electrode 7 and a lower electrode 8 were formed by vacuum evaporation.

In the silicon guided waveguide of this example, the cladding layer 3
When forming b, the B component diffuses near the surface of the core portion 2a,
As shown in FIG. 3, 14 layers are formed on the surface of the core portion 2a, which was originally n-type.

That is, a so-called p'''n junction is formed on the upper surface of the core part 2a.Therefore, the electrode 8i7 is connected to ten electrodes, and the lower electrode
When a forward bias is applied to the p0n junction with 2 as one pole, a large amount of carriers are injected into the core portion 2a, causing a change (reduction) in the refractive index in the injection region. The injection current density required for this purpose was about 1 kA/cm2. Such a phenomenon of refractive index change is extremely effective when utilizing a silicon guided waveguide as a component of an optical phase filter or an optical switch.

In this way, in Example 2, the quartz-based cladding layer 3
In addition to the effect of reducing the optical propagation loss of the silicon optical waveguide, b is also useful for forming a p9n junction and functionalizing the silicon guiding waveguide.

Regarding the appropriate range of B2O3 doping concentration in the cladding layer 3b, it has been confirmed that the discussion is approximately the same as the P2O5 doping concentration range in Bi Example 1.

Although the structure and manufacturing method of the silicon guided waveguide of the present invention have been described above with respect to two examples, the present invention is not limited to these examples. It is also possible to implement various modifications such as providing a p-type silicon core portion.

Alternatively, P2O and 8203 can be simultaneously contained as dopants in the silica-based glass crat layer. In this case, the total addition concentration of P2O5 and has (
The same discussion on the appropriate concentration range as discussed in Example 1 can be made regarding (mol%). In this case, the polarity of the diffusion layer on the upper surface of the silicon core portion is determined to be n-type or p-type, depending on whether the P component or the B component is predominant.

Figures 4(a) and 4(b) show the configuration of a silicon guided waveguide with an optical fiber connection guide groove as a third embodiment of the present invention, and Figure 4(a) shows the structure before the fiber is inserted. FIG. 4(b) is a sectional view after the fiber is inserted.

Here, thick quartz-based glass clad Ji! ! The process of forming the silicon optical waveguide core 2a embedded with the silicon optical waveguide core 3b is the same as that shown in FIGS.
A part of the silicon substrate 1 was removed, and a guide groove 9 for connecting an optical fiber was provided near the end of the core portion 2a. The width of this guide groove 9 is the outer diameter Cl of the single mode optical fiber IO.
It was set to 126 μm in accordance with 25 μl11). The depth of the guide groove 9 is such that when the optical fiber 1.0 is inserted into the guide TF49, the core part of this optical fiber matches the core part 2a of the silicon guided waveguide, as shown in FIG. 4(b). It shall be adjusted so that

When we actually measured the connection loss between the optical fiber and the silicon guided waveguide with the above configuration, it was about 3 dB.

Approximately 1 dB of this was Fresnel reflection loss due to the difference in refractive index between the silicon leading waveguide and the optical fiber.
A connection loss of less than 2 dB was achieved with a configuration in which Fresnel reflection was eliminated by applying anti-reflection coating to the end of the silicon pilot waveguide.

It is clear that a guide structure similar to the optical fiber connection guide described in Example 3 can be used not only for optical fibers but also for attaching optical devices such as light receiving and emitting elements to silicon guided waveguides.

Furthermore, if the grooves formed by removing a part of the silica-based glass cladding layer 3b are used in conjunction with the heteroepitaxial growth technology of compound semiconductors on silicon substrates, which has been rapidly progressing in recent years, It is also possible to serve as a growth guide groove when a compound semiconductor optical device is directly formed in the middle.

[Effects of the Invention] As explained above, in the present invention, by employing a silicon optical waveguide structure provided with a thick silica-based glass cladding layer, the predecessor 111i of the silicon optical waveguide, which has been a problem in the past, can be solved.
i loss can be significantly reduced.

Furthermore, according to the present invention, a silica-based glass crat layer containing P2O or B2O3 as a dopant that lowers the softening temperature of silica-based glass is formed through a glass particle deposition and transparency process using a flame hydrolysis reaction. By doing so, a thick crat layer can be easily formed without impairing the prior withdrawal effect of the silicon guiding waveguide.

Furthermore, in the present invention, it is also possible to impart functionality to the silicon leading waveguide by providing a pn junction in the silicon leading waveguide and applying a bias voltage thereto, if necessary.

Alternatively, since the silicon guiding waveguide core portion is covered with a thick silica-based glass cladding layer, there is an advantage that handling becomes easier.

Furthermore, according to the present invention, -i guide grooves can be formed in the silica-based glass cladding layer by a method such as reactive ion etching, thereby making it possible to easily connect an optical fiber to a silicon guiding waveguide or attach an optical element. It also has effects.

Due to the above effects, the silicon optical waveguide of the present invention and its manufacturing method are expected to occupy an important position not only in the above-mentioned optical integrated circuit application field but also in new guiding waveguide application fields such as so-called optical interconnection. Ru.

[Brief explanation of the drawing]

FIGS. 1(δ), (b), (c), and (d) are cross-sectional views showing an example of the manufacturing process of an embodiment of the silicon guided waveguide according to the present invention, and FIG. 3 is a cross-sectional view for explaining the details of the cross-sectional structure of a silicon guided waveguide; FIG. 3 is a cross-sectional view showing the cross-sectional structure of a silicon guided waveguide with a current injection part as a second embodiment of the present invention; and (b) are perspective views and cross-sectional views, respectively, showing the configuration of a silicon guided waveguide with an optical fiber connection guide as a third embodiment of the present invention. FIG. 3 is a cross-sectional view showing two examples of wave path cross-sectional structures. 4... Upper diffusion layer, 5... Lower diffusion layer, 6... Opening, 7... Electrode, 8... Lower electrode, 9... Optical fiber guide lO... Optical fiber. groove,

Claims (1)

[Claims] 1) a silicon substrate that also serves as a lower cladding layer; a silicon core portion disposed on the silicon substrate and having a lower free carrier concentration than the silicon substrate; and a silicon core portion disposed so as to embed the silicon core portion. , and a silica-based glass cladding layer containing a dopant that lowers the softening temperature of silica-based glass. 2) A diffusion layer having a free carrier concentration higher than that of the silicon core portion is provided on a surface of the silicon core portion that is in contact with the silicon substrate and a surface that is in contact with the silica-based glass cladding layer. 3) A step of epitaxially growing a silicon layer having a lower free carrier concentration than the silicon substrate on the silicon substrate, and removing an unnecessary portion of the silicon layer having a lower free carrier concentration to form a ridge-shaped silicon layer. The method is characterized by comprising a step of forming a core portion, and a step of forming a silica-based glass cladding layer containing a dopant that lowers the softening temperature of the silica-based glass so as to embed at least the ridge-shaped silicon core portion. Method for manufacturing silicon optical waveguide. 4) The step of forming the silica-based glass cladding layer includes depositing glass fine particles on the silicon substrate on which the ridge-shaped silicon core is formed, by a flame hydrolysis reaction of the glass-forming raw material gas so as to embed the ridge-shaped silicon core. and a step of heating the glass particles deposited on the silicon substrate together with the silicon substrate to turn the glass particles into transparent vitrification to form the silica-based glass cladding layer. 4. The method of manufacturing a silicon optical waveguide according to claim 3.