JPH0933741A - Quartz optical waveguide and method of manufacturing the same - Google Patents

Abstract

(57) [Abstract] (Correction) [Problem] Even if the core layer is continuously etched to obtain a silica optical waveguide having stable optical characteristics, the core layer has good reproducibility regardless of the number of batches. Provided is a method for manufacturing a silica-based optical waveguide, which is capable of controlling the etching depth. In a silica-based optical waveguide having a core pattern formed by reactive ion etching, the core pattern is formed by controlling the self-bias voltage during the reactive ion etching of the core layer to be constant. And a method for manufacturing the same.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silica optical waveguide and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, formation of a silica-based optical waveguide core has been
The following method of forming a core layer on the under clad layer and forming a core pattern is known. That is, in order to form the core layer, the flame deposition method on the Si substrate,
Under-cladding layer and core layer are formed by forming a porous film for under-cladding layer and a core porous film made of quartz glass containing a refractive index controlling additive (Ge or Ti etc.) Form (M. Kawati, Op
tical and Quantum Electronics, 22, 391-416, 199
0), or a core layer is formed on a quartz glass substrate that also serves as an underclad layer by an electron beam evaporation method using an evaporation material containing SiO ₂ and a refractive index control additive (such as Ti) (K.Imoto). Others, APPLIED OPTICS, Vol.26, No.19,
4214-4219, 1987).

The following method (method 1) is known as a method of forming a core pattern. A positive photoresist pattern having a thickness of 3 μm is formed on the core layer by photolithography, and the core pattern is formed by reactive ion etching using a mixed gas of CHF ₃ and C ₂ F ₆ (J. Allen, OPTICAL ENGINEERING). , Vol.32, No.5,
1011-1014, 1993).

Next, the following method (method 2) is known. A Ti layer is formed on the core layer by a vacuum deposition method, a photoresist pattern is formed by photolithography, unnecessary portions of the Ti film are removed to form a mask pattern for core pattern formation, and C ₂ H ₄ and C ₂ are formed. _A core pattern is formed by reactive ion etching using a mixed gas with ₂ F ₆ (T. Izawa, Appl. Phys. Lett, Vol. 38, No. 7, 3
91-416, 1990).

The following method (method 3) is also known. An amorphous Si film is formed on the core layer, a photoresist pattern is formed by photolithography,
An amorphous Si mask pattern for core pattern formation is formed by reactive ion etching using CBrF ₃ gas, and a core pattern is formed by reactive ion etching using a mixed gas of C ₂ H ₄ and C ₂ F _6. (M.
Kawati, Optical and Quantum Electronics, 22, 391 ~
416, 1990).

Further, the following method (method 4) is known. The WSi _X film was formed by RF sputtering on the core layer by photolithography to form a photoresist pattern, a WSi _X mask core pattern formation is formed by reactive ion etching using NF ₃ gas, and CHF ₃ A core pattern is formed by reactive ion etching using a mixed gas of C ₂ F ₆ (K.Imot
o Others, APPLIED OPTICS, Vol.26, No.19, 4214-4219, 1
987).

[0007]

However, the silica-based optical waveguide manufactured by the above-mentioned method of forming a core pattern has the following problems. In the forming method of Method 1, since the selection ratio, which is the ratio of the etching rate of the core layer and the etching rate of the photoresist that is the mask pattern for core pattern formation, is as small as 3 to 4 regardless of the gas species,
If the core layer is to be etched by 10 μm, the thickness of the photoresist needs to be 2.5 μm to 3.3 μm. However, when the photoresist layer is as thick as 2.5 μm to 3.3 μm, the cross-sectional shape of the photoresist becomes trapezoidal. When a core pattern is formed by reactive ion etching using a trapezoidal photoresist as a mask, the core pattern also becomes a trapezoidal shape. However, there are problems such as not being able to obtain the above, and causing polarization dependence.

As a means for solving the above problem, Method 2
Since the side faces of the metal mask and the side face of the core pattern can be formed vertically by forming the core pattern using the metal mask as described in 4 to 4, when the silica-based optical waveguide multiplexer / demultiplexer is manufactured, polarization dependence A good multiplexing / demultiplexing characteristic can be obtained. In particular, using WSi _X as a metal mask,
As the etching gas, CHF ₃ alone or CH
When a mixed gas of F ₃ and C ₂ F ₆ is used (method 4),
The selectivity, which is the ratio of the etching rate of the core layer and the etching rate of the WSi _x mask, is 20 or more, which is larger than that when Ti or amorphous Si is used. Since the selection ratio is large, the film thickness of the metal mask may be small, and the manufacturing cost can be significantly reduced.

However, as a metal mask, WSi _{X is used.}
Using, in the case of using a mixed gas alone gas or CHF ₃ and C ₂ F ₆ of CHF ₃ as the etching gas (method 4), when the reactive ion etching of the core layer in succession, the same time etching Even when it was carried out, there was a problem that the etching depth of the core layer was different for each batch. In a remarkable case, the etching depth of the core layer may be about half the set depth. Further, although CBrF ₃ is used as a gas for reactive ion etching of amorphous Si in the method 3, there is also a problem that the CBrF ₃ gas is a CFC regulated gas and therefore its use is restricted.

[0010]

The inventors of the present invention have completed the present invention by paying attention to the self-bias voltage as a result of detailed investigation of the above problems. That is, in the present invention, the core pattern is formed by controlling the self-bias voltage during the reactive ion etching of the core layer to be constant in the silica-based optical waveguide in which the core pattern is formed by the reactive ion etching. The silica-based optical waveguide is characterized in that the core pattern is formed by controlling the self-bias voltage during the reactive ion etching of the core layer to be constant. The manufacturing method is as follows. This will be described in more detail below.

FIG. 1 shows the change in self-bias voltage with cumulative etching time when reactive ion etching of the core layer is performed with CHF ₃ while keeping the high-frequency power constant at 300 W. As shown in FIG. 1, the self-bias voltage of the cathode to which high-frequency power is applied during etching becomes smaller with time. Further, when the etching is performed for the same time, the etching depth of the core layer becomes smaller in the later batch wafers.

It is known that the etching rate of SiO ₂ is related to the following equation (1) (Matsuo, 1st Dry Process Symposium, The Institute of Electrical Engineers of Tokyo, 1)
3, 1979). V ∝ I · exp (−E ₀ / E) (1) In the formula (1), V is the etching rate, I is the ion incident flow, E ₀ is the activation energy, and E is the self-bias voltage.

From the relation of the equation (1), it is understood that it is necessary to control the ion incident flow and the self-bias voltage in order to control the etching rate. Further, if the high frequency power of reactive ion etching is X, I∝X ^1/2 , E∝X
It is known that it can be approximated to ^1/2 . Therefore, the equation (1) is expressed as the equation (2). V = A * X1 ^{/ 2} * exp (-B / X1 / ² ) ... (2) However, in Formula (2), A and B are constants.

From the equations (1) and (2), the following four types of methods can be considered to control the etching rate to be constant. (A) The amount of incident ions and the self-bias voltage are sequentially controlled independently. (B) The amount of incident ions is kept constant and the self-bias voltage is sequentially controlled. (C) The self-bias voltage is kept constant and the amount of incident ions is sequentially controlled. (D) The ion incident amount and the self-bias voltage are kept constant at the same time. In normal reactive ion etching, it is general to control high frequency power as a control method for keeping the etching rate constant. It is considered that controlling the high-frequency power is controlling the etching rate according to the equation (2), and that in the case of reactive etching of SiO ₂ , the above (d) is performed. However, C
In the case of reactive ion etching of the core layer using a single gas of HF ₃ or a mixed gas of CHF ₃ and C ₂ F ₆ , even if the high frequency power is controlled to be constant, as shown in FIG. It changes with the cumulative etching time. Therefore, it was considered that the etching rate cannot be controlled to be constant due to the change in self-bias voltage.

From the above, in order to make the etching depth of the core layer constant, that is, to make the etching rate of the core layer constant, the above (a) to (c) are used.
It is considered to be feasible by controlling the above.
However, although (a) and (c) cannot be performed by reactive ion etching using a high frequency power source,
When the pressure is controlled to be constant, it is possible to approximate that the amount of incident ions is almost constant. Therefore, in reactive ion etching using a high frequency power source, the control of (b) above can be performed and self-bias CHF ₃ alone gas or CHF ₃ gas is controlled by controlling the voltage to be constant.
It is considered possible to control the etching rate of the core layer using the mixed gas of C ₂ and C ₂ F ₆ . Here, the self-bias voltage is controlled within a range of ± 10V with respect to the set value, and preferably ± 5V.

From the above, in the reactive ion etching step of the core layer of the silica-based optical waveguide, the etching rate of the core layer can be controlled by successively controlling the self-bias voltage while keeping the ion injection amount constant, so that the core layer can be controlled. It is possible to control the etching depth of.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Next, embodiments of the present invention will be described with reference to FIGS. Example First, on a silica glass substrate 1 having a diameter of 100 mm which also serves as an underclad layer, GeO ₂ which is a refractive index control additive is added.
A core layer 2 having a thickness of 8 μm was formed by an electric beam evaporation method using a SiO ₂ glass evaporation material containing Si (FIG. 2).
(A)). The concentration of the refractive index control additive of the core layer 2 is set in advance so that the refractive index of the core layer 2 is 0.3% higher than the refractive index of the quartz glass substrate 1 which also serves as the under cladding layer. The control additive concentration was adjusted.

Next, a core pattern forming WSi _X mask layer 3 having a thickness of 450 is formed on the core layer 2 by a sputtering method.
nm (FIG. 2B).

Next, in order to form the WSi _X mask pattern 5 for forming the core pattern, a positive resist pattern 4 was formed to a thickness of 500 nm on the WSi _X mask layer 3 for forming the core pattern by ordinary photolithography (see FIG. 2 (c)).

Next, in order to form the WSi _X mask pattern 5 for forming the core pattern, the WSi _X mask pattern 5 for forming the core pattern is formed by reactive ion etching using NF ₃ gas by the apparatus shown in FIG. Form. On the lower electrode 23 connected to the high frequency power source 22 in the chamber 21, the core layer 2 and the core pattern forming W are formed.
The substrate to be etched 7 made of the quartz glass substrate 1 on which the Si _X mask layer 3 and the resist pattern 4 are laminated is arranged, and the turbo molecular pump 24 and the oil rotary pump 25 connected to the chamber 21 are used to reach 5 × 10 ⁻⁵ Pa. Chamber 2
The inside of 1 was evacuated. After that, NF ₃ gas was introduced into the chamber 21 at 10 sccm, and the chamber 21 was kept at 0.3 Pa by an automatic pressure controller at 13.56 MH.
z, high frequency power was applied to the lower electrode 23 while controlling the high frequency power to be constant at 20 W, and reactive ion etching was performed for 10 minutes to form a WSi _X mask pattern 5 for core pattern formation ( FIG. 2D).

Next, the photoresist pattern 4 remaining on the core pattern forming WSi _X mask pattern 5 is removed. A substrate 7 to be etched made of a quartz glass substrate 1 in which a core layer 2, a WSi _X mask pattern 5 for forming a core pattern 5 and a photoresist pattern 4 are laminated on a lower electrode 23 connected to a high frequency power source 22 in a chamber 21. 5 × 10 ⁻⁵ Pa by a turbo molecular pump 24 and an oil rotary pump 25 which are arranged and connected to the chamber 21.
The chamber 21 was evacuated to. Then, O ₂ gas was introduced into the chamber 21 at 50 sccm, and the chamber 21 was kept at 0.9 Pa by an automatic pressure controller.
High frequency power was applied to the lower electrode 23 while controlling the high frequency power to be constant at 3.56 MHz and 80 W, and reactive ion etching was performed for 5 minutes to remove the photoresist pattern (FIG. 2). (E)).

Next, a core pattern 6 is formed using the WSi _X mask pattern 5 for forming a core pattern. Lower electrode 2 connected to high frequency power supply 22 in chamber 21
A substrate 7 to be etched made of a quartz glass substrate 1 on which a core layer 2 and a WSi _X mask pattern 5 for forming a core pattern are laminated is arranged on the substrate 3, and a turbo molecular pump 24 and an oil rotary pump 25 connected to a chamber 21 are used. 5x
The chamber 21 was evacuated to 10 ⁻⁵ Pa. After this, C
HF ₃ gas was introduced into the chamber 21 at 100 sccm, and the inside of the chamber 21 was adjusted to 0.3 by an automatic pressure controller.
Keep at Pa, 13.56MHz, self-bias voltage is 1
The lower electrode 23 while controlling the voltage to be constant at kV
A high frequency power was applied to the substrate and reactive ion etching was performed for 120 minutes to form the core pattern 6 (FIG. 2).
(F)). At this time, the selection ratio, which is the ratio of the etching rate of the core layer 2 and the etching rate of the core pattern forming WSi _X mask pattern 5, was 25.

Next, the core pattern forming WSi _X mask pattern 5 remaining on the core pattern 6 is removed. On the lower electrode 23 connected to the high frequency power source 22 in the chamber 21, the core pattern 6 and the core pattern forming W are formed.
A substrate 7 to be etched made of a quartz glass substrate 1 on which a Si _X mask pattern 5 is laminated is arranged, and a turbo molecular pump 24 and an oil rotary pump 25 connected to a chamber 21.
Then, the chamber 21 was evacuated to 5 × 10 ⁻⁵ Pa. Then, NF ₃ gas was introduced into the chamber 21 at 10 sccm, and the chamber 21 was adjusted by an automatic pressure controller.
While maintaining the inside at 0.3 Pa, high frequency power is applied to the lower electrode 23 while controlling the high frequency power to be constant at 13.56 MHz and 20 W, and reactive ion etching is performed for 10 minutes to form a core pattern. The WSi _X mask pattern 5 for use was removed (FIG. 2G).

The core pattern was manufactured by reactive ion etching for 10 batches by the above method, and the etching depth of the core layer was measured by dimension measurement from the shape observation of a scanning electron microscope. FIG. 3 shows the etching depth of the core layer in the production of 10 batches. No decrease in etching depth was observed regardless of the number of batches.

Comparative Example For comparison, the same process as in Example was carried out except that the high frequency power was kept constant at 300 W in the step of forming the core pattern 6 using the WSi _X mask pattern 5 for forming the core pattern. In the process of forming this core pattern 6,
The self-bias voltage gradually changed from 730V to 160V. Further, the selection ratio, which is the ratio of the etching rate of the core layer 2 and the etching rate of the core pattern forming WSi _X mask pattern 5 at this time, gradually changed from 25 to 13. FIG. 4 shows the etching depth of the core layer in the production of 10 batches. As the number of batches increased, the etching depth decreased.

[0026]

According to the present invention, even if the core layer is continuously etched, the etching depth of the core layer can be controlled with good reproducibility regardless of the number of batches, and the quartz system can be stably used. Optical waveguides can be manufactured. Furthermore, by using the silica-based optical waveguide formed by the above manufacturing method, an optical component having stable optical characteristics can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a change in self-bias voltage according to cumulative etching time.

FIG. 2 is a schematic view showing a core layer etching process.
(A)-(g) is a cross-sectional schematic diagram in each process.

FIG. 3 is a diagram showing the etching depth of the core layer of each batch in the example of the present invention.

FIG. 4 is a diagram showing an etching depth of a core layer of each batch in a comparative example of the present invention.

FIG. 5 is a schematic view of a reactive ion etching apparatus.

[Explanation of symbols]

1 quartz glass substrate 2 core layer 3 WSi _X mask layer for core pattern formation 4 resist pattern 5 WSi _X mask pattern for core pattern formation 6 core pattern 7 substrate to be etched 21 chamber 22 high frequency power supply 23 lower electrode 24 turbo molecular pump 25 oil rotation pump

Claims (2)

[Claims] 1. In a silica-based optical waveguide having a core pattern formed by reactive ion etching, the core pattern is formed by controlling the self-bias voltage during the reactive ion etching of the core layer to be constant. A silica-based optical waveguide characterized by the following. 2. A quartz optical waveguide manufacturing method for forming a core pattern by reactive ion etching, wherein the self-bias voltage during the reactive ion etching of the core layer is controlled to be constant. Of optical system optical waveguide.