JPH1164658A - Production of optical waveguide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lessen the contamination of silicon substrates by manufacturing gas introducing pipes at the time of oxidation of single crystal silicon in a process for producing optical waveguides which forms oxidized films of >=5 μm by oxidizing the silicon substrates. SOLUTION: This process produces the optical waveguides by including a stage for forming the oxidized films by oxidizing the silicon substrates 2 arranged within a furnace core tube 1 by the steam generated by the reaction of the gaseous hydrogen and gaseous oxygen respectively introduced by a gaseous hydrogen introducing pipe 4 and a gaseous oxygen introducing pipe 5. At this time, at least the front ends of the gaseous hydrogen introducing pipe 4 and the gaseous oxygen introducing pipe 5 are respectively formed of the single crystal silicon.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical waveguide, and more particularly, to a method for preventing contamination by impurities such as particles when forming a silicon oxide film (quartz transparent film) used for an optical waveguide device for optical communication. The present invention relates to a method for manufacturing a prevented optical waveguide.

[0002]

2. Description of the Related Art An optical waveguide is usually first formed on a silicon or quartz substrate by 10 to 10 minutes.
A quartz film having a thickness of 20 μm is deposited or formed (this layer is referred to as an under clad), and a quartz film having a higher refractive index is deposited on the under clad for 5 to 10 μm (this layer is referred to as a core). After a pattern that allows light to pass through the core is formed by etching or the like, a quartz film having a refractive index smaller than that of the core is deposited on the core (this layer is referred to as over cladding). The device that splits and switches light according to the light is called an optical waveguide or a silica-based optical waveguide.

Conventionally, methods for producing a quartz glass thin film on a silicon substrate include a CVD method, a vapor deposition method, a flame deposition method, a sol-gel method, a high pressure oxidation method and the like.

[0004] The high pressure oxidation method is a method of oxidizing a silicon substrate in an oxidizing atmosphere at a pressure higher than normal pressure.

According to this method, a dense and pure quartz film can be obtained, so that it has excellent electrical insulation properties, and since both surfaces of the substrate are oxidized, the oxidized substrate is less likely to warp. . Therefore, of the quartz films used for the optical waveguide, the high-pressure oxidation method is used for the above-mentioned under clad quartz film.

However, in this method, the reaction for oxidizing the silicon substrate is very slow, and the oxidation rate is 1/2 power of the thickness of the quartz film. Requires hundreds to thousands and hundreds of hours.

Therefore, in order to accelerate the reaction, the oxidizing atmosphere may be an atmosphere containing water vapor (literature: VLSI Technology second).
edition p. 106). In addition, there is a method of increasing the pressure of an oxidizing atmosphere (Reference: VLSI Tec)
hnology second edition p.
121, JP-A-52-154360, etc.).

Usually, to generate steam, hydrogen and oxygen are introduced into a reaction tube through a gas introduction tube.
In general, high-purity quartz is used to prevent metal impurities and the like from being mixed. However, quartz is gradually crystallized in the generated water vapor by metal impurities of several ppm or less contained in quartz or contained in gas, particularly Al and alkali metals, via OH groups contained in quartz. It becomes cloudy. The cloudy portion is gradually peeled off from the surface by heat, and floats as particles of 1 to several μm in the reaction tube. It adheres to the silicon substrate in the reaction tube as dust of about 1 μm.

Particularly, in order to form an oxide film having a thickness of 5 μm or more used for an optical waveguide, as described above, the reaction time is several hundred to several hundreds.
Since it lasts for several thousand and several hundred hours, the probability of the adhesion is overwhelmingly higher than that of the semiconductor insulating film. In addition, the dust on the surface becomes a light scattering center when the device is used as an optical waveguide device, and becomes a factor for attenuating light.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has been made in consideration of the above circumstances. Therefore, when an oxide film is formed on a silicon substrate used for an optical waveguide, contamination by impurities such as particles can be prevented as much as possible. An object of the present invention is to provide a method for manufacturing a waveguide.

[0011]

Means for Solving the Problems and Embodiments of the Invention As a result of intensive studies to achieve the above object, the present inventor has found that at least the tip portions of the hydrogen gas introduction tube and the oxygen gas introduction tube are made of single-crystal silicon. Has been found to be effective in suppressing particles.

That is, in the high-pressure oxidation method, as described above, a silicon substrate is inserted into a quartz furnace tube, and a pressure of 1 atm or more is pressurized with nitrogen or argon, and the heater around the furnace tube is heated. This method raises the temperature to 700 ° C. or higher, and then causes hydrogen and oxygen to pass through a nozzle into a quartz furnace core tube to react those gases into steam, and oxidize the silicon substrate using the steam. In this case, since the reaction heat generated when hydrogen and oxygen react with each other is large, the temperature inside the furnace tube rises by several tens degrees or more than the temperature controlled by the heater. In particular, the tip of the nozzle rises more than one hundred degrees. At this time, a quartz tube is conventionally used as a material of the nozzle, but the quartz tube is exposed to high-temperature steam, and is gradually eroded by steam through an OH group contained in the quartz. The tip of the tube comes off. Although it depends on the type of quartz in the quartz tube, the quartz tube manufactured by the melting method is more easily reacted than the quartz tube of the flame method because it contains OH groups of 100 ppm or more. The flame method is several tens pp
Although only OH groups of m or more are contained, if the OH groups are small, there is a disadvantage that it is difficult to process the tube.

If the impurities contained in the quartz or the impurities attached to the surface include alkali metals such as Al, Na, and K, the quartz crystallizes in a steam atmosphere at a high temperature and becomes cloudy. As a result, the quartz tube is separated in a minute shape.

This floats in the core tube of the quartz furnace and adheres to the surface of the silicon substrate to cause particles.

In response to such a problem, the present inventors have suppressed the reaction between the nozzle and water vapor by using a single-crystal silicon nozzle for introducing hydrogen and oxygen in a furnace for high-pressure oxidation of a silicon substrate. It has been found that particles generated on the silicon substrate can be suppressed.

Single crystal silicon has a high purity with an impurity amount of 1 ppm or less, a diameter of 10 to 20 mm and a length of 100 ppm.
As long as the shape of the nozzle tube is about 200 mm, it is a size that can be easily processed from a normally produced semiconductor ingot. Processing does not have any problem with the accuracy obtained by ordinary machining. In addition, even if the surface is contaminated with metal during processing, it is possible to remove surface contamination by etching with HF or the like. Further, since the melting point is 1420 ° C., there is no problem in heat resistance. In addition, the reactivity with water vapor is the same as that of the silicon substrate set in the quartz furnace core tube, so that even when exposed to water vapor for several hundred hours, only an oxide film of less than 100 μm is formed on the surface. The quartz film does not become cloudy. Further, since the generated oxide film has a very small number of OH groups of several tens of ppm or less, the reactivity is low.

Therefore, it is possible to suppress the generation of dust from the nozzle having a high temperature, and it is possible to greatly reduce particles adhering to the silicon substrate.

Hereinafter, the present invention will be described in more detail.
The present invention includes a step of forming an oxide film by oxidizing a silicon substrate disposed in a furnace core tube with water vapor generated by a reaction of hydrogen gas and oxygen gas introduced by a hydrogen gas introduction tube and an oxygen gas introduction tube, respectively. In the method for manufacturing an optical waveguide including at least one of the hydrogen gas introduction pipe and the oxygen gas introduction pipe, at least the tip portions are each made of single-crystal silicon.

Here, as a method for oxidizing the silicon substrate to form an oxide film (quartz transparent film), an apparatus as shown in FIG. 1 can be used.

That is, in FIG. 1, 1 is a furnace core tube,
The silicon substrate 2 is disposed therein. This furnace core tube 1
A heater module 3 is disposed outside the furnace so as to cover the furnace core tube 1. Further, one end surface of the furnace core tube 1 is sealed, and the one end surface is airtightly penetrated to form a hydrogen gas introduction pipe (nozzle).
The respective distal ends of the oxygen gas introduction pipe (nozzle) 5 and the nitrogen gas introduction pipe (nozzle) 6 are inserted into the furnace core tube 1. The other end opening of the furnace core tube 1 is hermetically closed by a cap 7. Further, a tank 10 provided with a water-cooling jacket 9 is connected to the furnace core tube 1 via an exhaust pipe 8, and water vapor from inside the furnace core pipe 1 passes through the exhaust pipe 8 and is connected to the tank 1.
The gas is exhausted into the inside of the cooling water tank 0 and is cooled and stored by the water cooling jacket 9. Water vapor that cannot be collected in the tank 10 is discharged from the discharge pipe 11 to the outside of the system.

In the present invention, the hydrogen gas introduction pipe 4 and the oxygen gas introduction pipe 5 which are formed at least at the end portions thereof, in particular, the portions to be inserted into the furnace core tube 1 are made of single crystal silicon. If necessary, the entirety of the hydrogen and oxygen gas introduction pipes 4 and 5 can be formed of single crystal silicon. The nitrogen gas introduction pipe 6 may be formed of quartz, but if necessary, at least its tip,
In particular, the part or the whole inserted into the furnace core tube 1 can be formed of single crystal silicon.

In the present invention, the conditions for oxidizing the silicon substrate by the high-pressure oxidation method can be adopted in a conventional manner, and the oxidation can be performed at a normal temperature of 700 to 1200 ° C. and a pressure of 1.1 to 25 atm. 5 μm or more, preferably 10
An oxide film of about 20 μm can be formed.

Although the under cladding is formed by this method, the subsequent production of the core, pattern and over cladding can be performed by a conventional method.

[0024]

According to the present invention, in a method of manufacturing an optical waveguide for oxidizing a silicon substrate to form an oxide film having a thickness of 5 μm or more, a gas introduction pipe for oxidizing is made of single crystal silicon. Accordingly, contamination of the silicon substrate can be reduced.

[0025]

EXAMPLES The present invention will be described below in detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

EXAMPLE A silicon substrate was oxidized using the oxidation furnace shown in FIG. In this case, this oxidation furnace
In a 50 mmφ, 1500 mmL tantalum heater module 3, a 200 mmφ, 1000 mmL quartz furnace core tube 1 is placed concentrically. Heater module 3
Is a heater divided into three equal parts, which can be soaked within ± 1 ° C. 4 ″ φ × 1 mm thick silicon substrate 2 was placed in furnace core tube 1
Set vertically to. Two 10 mmφ, 200 mmL nozzles 4 and 5 are inserted into the tail of the quartz furnace core tube 1. Although not shown, the nozzles are provided with piping so that hydrogen flows from the nozzle 4 and oxygen flows from the nozzle 5, respectively. Further, an exhaust pipe 8 for exhausting the generated water vapor is provided at the bottom of the furnace core tube. The exhausted steam is cooled by a water cooling jacket 9 and stored in a tank 10 as water.
Water vapor that cannot be collected in the tank 10 flows out of the system through the discharge pipe 11. An airtight quartz cap 7 is attached to the head of the quartz furnace core tube 1 so that the pressure inside the furnace core tube can be increased.

Here, the nozzle 4 is a double tube having an outer diameter of 15 mm, an inner diameter of 10 mm, and a length of 200 mm, and the distal end portion has a shape whose inner diameter is reduced to 5 mm. This is cut and processed from an ingot of a floating method (FZ method) which does not contain a 9N level doping material used for a semiconductor. The machined surface is # 60
After finishing with a machined surface roughness of about 0, the surface is etched to a thickness of 500 μm by immersion in a 25% HF aqueous solution. In an argon or nitrogen atmosphere,
Baking is performed at a temperature of about 0 ° C. Also in the nozzle 5, the same shape is finished by the same processing method.

Next, hydrogen and oxygen at 5 atm were supplied into the quartz furnace core tube 1 through the respective nozzles 4 and 5 at 3.6 L / mi.
n, 2.0 L / min. Therefore, the inside of the furnace core tube is 5
Maintained at atmospheric pressure.

As the nitrogen nozzle 6, a silicon nozzle treated in the same manner as the nozzles 4 and 5 was used.

The heater temperature was set so that the temperature in the furnace tube 1 was 1000 ° C. By setting the heater close to the gas inlet of the three divided heaters at 800 ° C. or higher, hydrogen and oxygen are ignited and turned into steam.

The silicon substrate in the furnace core tube 1 was reacted with the steam for about 200 hours to form an oxide film of about 20 μm.

After the reaction, when the nozzles 4 and 5 were taken out, the surface was oxidized.
It was only 5 μm, and no discoloration or clouding was observed at the tip.

When the oxidized 4 ″ φ silicon substrate was observed with a halogen lamp of 200,000 Lx, only 30 particles having a size of 1 μm or more were found in a plane other than the surrounding 2 mm. FIG. This shows the state of particles attached to the substrate.

Comparative Example Using the same apparatus as in FIG. 1, the two nozzles 4 and 5 have the same shape but are made of high-purity quartz having an OH group content of about 100 ppm. Processing is the same, but fire polished (to make the surface transparent by dusting with a hundred thousand degrees of oxyhydrogen flame).

In the same manner as in the embodiment, the silicon substrate is
In a quartz furnace core tube heated to 00 ° C and pressurized to 5 atm,
Hydrogen and oxygen at 5 atm were reacted and oxidized with steam.

The oxidation time is about 200 hours, and the obtained oxide film thickness is 20 μm.

After the reaction, when the nozzles 4 and 5 were taken out, the inner surface was transparent, but the tip portion was cloudy with a thickness of several hundred μm over the periphery of the opening 5 mm and the length 50 mm. It came off when touched by hand.

Further, the oxidized 4 ″ φ silicon substrate is
Observation with a halogen lamp of 10,000 Lx shows that particles of 1 μm or more
About 0 pieces were uniformly attached. Observation of the particles with an electron microscope revealed that a number of 0.2 μm spherical particles overlapped. FIG. 3 shows a state of particles attached to the silicon substrate.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of an oxidation furnace used for carrying out the present invention.

FIG. 2 is an explanatory diagram showing a state of particles attached to a silicon substrate in an example.

FIG. 3 is an explanatory diagram showing a state of particles attached to a silicon substrate in a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Furnace tube 2 Silicon substrate 3 Heater module 4 Hydrogen gas introduction tube (nozzle) 5 Oxygen gas introduction tube (nozzle) 6 Nitrogen gas introduction tube (nozzle) 7 Cap 8 Exhaust tube 9 Water cooling jacket 10 Tank 11 Discharge tube

Claims (1)

[Claims] 1. A silicon substrate disposed in a furnace core tube,
The method for manufacturing an optical waveguide, comprising the step of forming an oxide film by oxidizing with water vapor generated by a reaction of hydrogen gas and oxygen gas introduced by a hydrogen gas introduction pipe and an oxygen gas introduction pipe, respectively, A method for manufacturing an optical waveguide, wherein at least the tip portions of the gas introduction pipes are each formed of single-crystal silicon.