JP2001033644A - Manufacture of optical waveguide film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of an optical waveguide film prevented from being contaminated by impurities or particles when an oxide film having 10 μm or more thickness is formed on a silicon substrate as the optical waveguide. SOLUTION: When an oxide film having a thickness of 10 μm or more is formed by a thermal oxidation method on a surface of a silicon substrate, an oxidation process is interrupted for a time at the time when its film thickness is grown upto 5 μm or more, the oxide film grown on the surface is etched removedly using a hydrofluoric acid solution to be brought into a thickness of 0.5 μm or less, and an oxide film is grown again on it to form a desirable thickness of oxide film.

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 film used for an optical waveguide device for optical communication, in particular, a transparent quartz film.

[0002]

2. Description of the Related Art As a substrate of a waveguide device for optical communication,
A quartz substrate or a silicon substrate is used. Above all, silicon substrates have good thermal conductivity, various processing methods,
For example, in addition to being excellent in surface workability such as etching, oxidation, and deposition, the diameter is further increased in production, and it can be obtained at low cost.

A method for growing a quartz glass thin film on a silicon substrate includes a CVD method, a vapor deposition method, a flame deposition method, a sol-gel method, a high-pressure oxidation method, and the like. The high-pressure oxidation method is a method of oxidizing a silicon substrate in an oxidizing atmosphere under a pressure higher than normal pressure (literature: VLSI technology seco).
nd edition p.121, JP-A-52-154360).
In this method, a silicon substrate is set in a furnace core tube made of quartz, and heated under a pressure of 1 atm or more in a nitrogen or argon atmosphere by a heater disposed around the furnace core tube.
This is a method in which the temperature is raised to a degree or more, and, for example, hydrogen and oxygen are supplied from a nozzle into the furnace core tube, and the silicon substrate is oxidized by steam gas generated by the reaction. A pure quartz film is obtained. In addition, this high-pressure oxidation method has an advantage that a quartz film is formed on both surfaces of a substrate by oxidation, and thus the substrate after oxidation is hardly warped. For this reason, the high-pressure oxidation method is used for forming an underclad quartz film having a thickness of 10 μm or more, among the quartz films used for the optical waveguide.

However, in this method, the reaction rate for oxidizing the silicon substrate is extremely slow, ie, 1/2 power of the thickness of the quartz film. It takes ~ 1,000 and several hundred hours. Further, in the high-pressure oxidation method, high-purity quartz is usually used for a furnace core tube in which a silicon substrate is placed in order to prevent contamination of metal impurities and the like, but still contained in the quartz in steam. O
With the H group interposed, the quartz is gradually dissolved in water vapor by several ppm or less of metal impurities contained in the quartz or contained in the reaction gas, particularly Al and alkali metals, and the dissolved quartz is changed by a temperature change. It precipitates on the inner wall of the furnace tube and on the silicon substrate and adheres as particles.

[0005] The dissolution and precipitation reaction of quartz depends on the concentration and pressure of water vapor and the time of exposure to water vapor. In addition, particles precipitated on the silicon substrate by this reaction gradually grow while being exposed to water vapor.
At the beginning of the deposition, they adhere in a spherical soot state of about 0.5 μm, but as time elapses, the particles aggregate and grow in a lump. Therefore, when an oxide film having a thickness of 20 μm is formed, particles grow to approximately 2 to 3 μm. In particular,
In order to form an oxide film having a thickness of 10 μm or more used as an optical waveguide, as described above, since the reaction time is several hundred to several thousand hours, the probability that particles will adhere is overwhelmingly higher than that of a semiconductor insulating film. High. In addition, when the particles attached to the surface have a size of several μm, they become light scattering centers in an optical path when a device is formed, and also serve as a factor for attenuating light.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to manufacture an optical waveguide film in which contamination such as impurities or particles is prevented when an oxide film having a thickness of 10 μm or more is formed on a silicon substrate. It is to provide a method.

[0007]

According to a method of manufacturing an optical waveguide film of the present invention, when an oxide film having a thickness of 10 μm or more is formed on a silicon substrate surface by a thermal oxidation method, the oxide film has a thickness of 5 μm.
m, the oxidation process is suspended, and the oxide film grown on the surface is reduced in thickness with an aqueous solution of hydrofluoric acid.
Etching is removed until the thickness becomes 0.5 μm or less, and an oxide film is grown thereon again to form an oxide film having a desired thickness. Further, a step of removing a part of the oxide film grown on the surface by etching and growing an oxide film thereon again may be performed a plurality of times. As described above, by partially removing the oxide film by etching and growing the thermal oxide film again, the contamination of the silicon substrate is reduced.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing an example of an oxidation furnace used for growing an oxide film for an optical waveguide on a silicon substrate by the manufacturing method of the present invention. In the figure,
A plurality of silicon substrates 1 are set in a holder 2 and housed in a furnace core tube 3 made of quartz. Around the furnace tube 3, a heater 4 for heating the furnace tube 3 is installed coaxially with the furnace tube 3. The heater 4 is configured by a heater module divided into three equal parts so as to uniformly heat the inside of the furnace core tube 3. Further, the furnace core tube 3 is kept airtight by a quartz cap 5 attached to one end thereof.
The pressure inside the furnace tube is increased. Two nozzles are inserted into the tail of the furnace core tube 3, hydrogen and oxygen are separately supplied from each nozzle, and steam is generated by a thermal reaction in the furnace core tube. The generated steam is discharged out of the system through the exhaust pipe 6.

FIG. 2 shows the relationship between the oxide film thickness and the oxidation time in the high-pressure oxidation method. The oxidation time in the figure is the cumulative oxidation time in which the substrate once oxidized was partially removed by etching and then oxidized again. The solid line curve was obtained in an Ar 94% atmosphere, and the broken line curve was obtained in an N ₂ 74% atmosphere. It is. From the figure, it can be seen that even if the oxide film is grown intermittently, the oxide film thickness is proportional to one half of the oxidation time obtained from the theory of diffusion control. Therefore, even if the oxidation step is interrupted until the desired oxide film is obtained and the oxidation is repeated, the cumulative oxidation time required for the oxidation does not change and does not depend on the number of interruptions.

The mechanism of generation of particles adhering to the silicon substrate is as described above.
Since the particle generation reaction proceeds simultaneously with the oxidation reaction of the silicon substrate, the generation of particles can be suppressed by lowering the water vapor concentration and the pressure. However, this is not economical because the oxidation rate of the silicon substrate is suppressed and the oxidation time is lengthened.

Since the particles are dissolved and precipitated from the quartz furnace core tube, the main component is quartz. Therefore,
These particles can be removed by etching the oxide film with a hydrofluoric acid aqueous solution. In addition, particles grow gradually with the passage of oxidation time,
The growth rate depends on the water vapor concentration and the pressure. For example, when the oxidation reaction is performed at 5 atm, 1,000 ° C., and a water vapor concentration of 90% for 50 hours, the particles grow to a size of 0.5 μm. The oxide film thickness is 10 μm.

In order to remove particles having a size of 0.5 μm, the amount of etching away from the oxide film surface in the thickness direction by a hydrofluoric acid aqueous solution may be set to 0.2 μm or less, preferably 0.1 to 0.2 μm. Since the thickness of the oxide film is reduced by the etching, an oxide film corresponding to the reduced thickness must be grown again by the high-pressure oxidation method. If the etching removal amount is too large, the oxidation time becomes longer. Furthermore, the etching removal is not necessarily performed uniformly over the entire surface of the silicon substrate, and in order to prevent etching unevenness, the etching removal amount in the thickness direction is desirably 0.2 μm or less from the surface.

The etching is preferably performed when the oxide film is grown to a thickness of 5 μm to 15 μm, preferably about 10 μm. Particles grow with the progress of oxidation, but the size of these particles grows
The thickness is about 0.5 μm when the oxide film is grown to a thickness of 10 μm. In addition, since particles adhere to the surface of the oxide film in a soot shape, the dissolution rate with respect to hydrofluoric acid is higher than that of a dense oxide film, and if the particle size is about 0.5 μm, the particle has an oxide film of 0.2 μm. Dissolves faster than μm etched. The oxide film has a thickness of 15μ.
m, the particles have grown to a size of 1.0 μm or more, and cannot be removed by etching of about 0.2 μm. Conversely, the particles taken into the oxide film are exposed on the surface, and the particles are grown. It is not preferable because it increases.

Even if a clean oxide film is once obtained by etching, if a thicker thermal oxide film is grown by a high-pressure oxidation method or the like, particles adhere to the surface of the oxide film similarly. In such a case, for example, after the etching process is performed when the oxide film thickness is 10 μm, the oxide film is grown again, and when the thickness becomes 15 μm, the etching process is performed again to further clean. Oxide film can be obtained. The time required for growing the oxide film from 10 μm to 15 μm requires the same oxidation time as the growth of an oxide film having a thickness of 10 μm because the oxide film is generated at a speed proportional to a half power of the oxidation time. With the above measures, particles having a diameter of 3.0 μm or less can be suppressed to 100 particles / plane or less on a silicon substrate having a diameter of 4 ″ φ.

[0015]

The present invention will be described in more detail with reference to the following Examples and Comparative Examples. (Example 1) A cylindrical Kanthal heater module having a diameter of 400 mmφ and a length of 2,500 mm was coaxially mounted with a diameter of 35 mm.
A quartz core tube having a diameter of 0 mm and a length of 2,000 mm was set. The heater module is divided into three equal parts, and the inside of the furnace tube can be soaked within ± 1 ° C. A silicon substrate having a diameter of 4 ″ φ and a thickness of 1 mm was set on a quartz board in a furnace tube. Two nozzles with a diameter of 10 mmφ and a length of 200 mm were inserted into the tail of the quartz furnace core tube, and one nozzle was supplied with 3.6 l / min of hydrogen and the other nozzle was supplied with 2.0 l / min of oxygen to cause a thermal reaction. Steam was generated in the tube.

As the high-pressure oxidation conditions, a heater set temperature of 1, 0
At 10 ° C., the pressure in the furnace tube was set to 5 atm, and an oxidation film having a thickness of 10 μm was grown over an oxidation time of 50 hrs. When the number of particles was measured with a foreign substance inspection device (manufactured by Hitachi Electronics Engineering), 120 particles having a size of 0.3 to 0.5 μm, 0.5
20 to 2.0 μm, 10 to 2.0 to 3.0 μm, and 5 to 3.0 μm or more. Therefore, when the oxide film was etched with a 1.0% hydrofluoric acid aqueous solution for 15 min.
μm was etched. Foreign matter inspection machine (same as above)
When the number of particles was measured with 20, 20 particles with a size of 0.3 to 0.5 μm, 15 with 0.5 to 2.0 μm, 2.0 to 3.0 μm
15 and 3.0 μm or more decreased to 8. When this substrate was again oxidized for 150 hrs under the above oxidation conditions, an oxide film having a thickness of 20 μm was obtained. At this time, the number of particles was 35, 0.3 to 0.5 μm, and 0.5 to 0.5 μm.
30 pieces of 2.0 μm, 25 pieces of 2.0-3.0 μm, 10 pieces of 3.0 μm or more
Became individual. The results are shown in Table 1.

(Comparative Example 1) An oxide film was grown on the same apparatus and under the same conditions as in Example 1 except that the oxidation time was changed to 150 hours. The thickness of the grown oxide film at this time is 16 μm.
When the number of particles on the surface of the oxide film was measured with a foreign matter inspection device (same as above), 30 particles with a size of 0.3 to 0.5 μm were found.
70, 0.5-2.0μm, 50 2.0-3.0μm, 3.0μ
m or more were 10 pieces. Then, when the oxide film was etched with a 1.0% hydrofluoric acid aqueous solution for 15 minutes, the surface was etched by 0.15 μm in the thickness direction from the surface. When the particles were measured in the same manner, 10 to 0.3 μm
65, 0.5-2.0μm, 75 2.0-3.0μm, 3.0μ
The number of particles increased to 10 or more depending on the size of the particles.

The average particle size is 2.0 μm.
First, particles having a size of 2.0 μm cannot be removed with an etching amount of 0.2 μm. But 0.3-0.
Those having a size of 5 μm are reduced because they can be removed by etching. Also, the reason why the number of particles having a diameter of 2.0 to 3.0 μm has increased is considered that the particles buried inside by the etching were exposed on the surface. The etching amount is 1.0
If the particle size is increased to μm, particles may be further removed. However, in order to grow an oxide film of 1 μm corresponding to the removed thickness, the thickness of the oxide film at this point (16 μm) is used.
It takes 10 to 20 hrs and the oxidation time is very long, which is not very economical. When this substrate was oxidized again for 50 hrs under the above-mentioned oxidation conditions, an oxide film having a thickness of 20 μm was obtained. When the number of particles was measured, 0.3-0.5μ
25 with a size of m, 80 with a size of 0.5 to 2.0 μm, 2.0 to
There were 85 pieces of 3.0 μm and 15 pieces of 3.0 μm or more. The results are shown in Table 1.

[0019]

[Table 1]

(Example 2) An oxide film was grown on the same apparatus and under the same conditions as in Example 1 except that the oxidation time was changed to 50 hours. The thickness of the grown oxide film at this time is 10 μm.
When the number of particles was measured with a foreign substance inspection machine, 95 particles having a size of 0.3 to 0.5 μm were obtained, and 95
Were 25, 10 to 2.0 to 3.0 μm, and 5 to 3.0 μm or more. When this oxide film was etched with a 1.0% hydrofluoric acid aqueous solution for 15 min, it was etched from the surface in the thickness direction by 0.15 μm. Pcs, 0.5-2.0μm
Were 18 pieces, 12 pieces of 2.0-3.0 μm, and 5 pieces of 3.0 μm or more, which were greatly reduced.

This substrate is again subjected to the above oxidation conditions for 50 hours.
When oxidized, an oxide film with a thickness of 15 μm was obtained, and the number of particles at this time was 0.3 to 0.5 μm.
75 pieces, 0.5-2.0 μm 35 pieces, 2.0-3.0 μm 30 pieces, 3.0
The number was 10 μm or more. Therefore, the oxide film is again set to 1.0
% Hydrofluoric acid aqueous solution for 15 min.
μm etched, the number of particles is 0.3-0.5μm
15 pieces of size, 15 pieces of 0.5-2.0 μm, 2.0-3.0
The number of μm was 10, and the number of 3.0 μm or more was 5, which was significantly reduced. When this substrate was oxidized again for 50 hrs under the above oxidation conditions, an oxide film having a thickness of 20 μm was obtained.
m was 20 pieces, 2.0 to 3.0 μm was 15, and 3.0 or more was 10 pieces. The results are shown in Table 2.

(Comparative Example 2) An oxide film was grown on the same apparatus and under the same conditions as in Example 1 except that the oxidation time was changed to 200 hours. The thickness of the oxide film grown at this time is 20 μm.
Particles of this oxide film were measured with a foreign matter inspection device (same as above), and 20 particles having a size of 0.3 to 0.5 μm were obtained.
60 pieces of 5-2.0 μm, 80 pieces of 2.0-3.0 μm, and 20 pieces of 3.0 μm or more. The results are shown in Table 2. The number and size of the particles are different from those of the first embodiment because the particles grow with the oxidation time and the size increases. This is because, at first, a spherical soot with a size of 0.3 to 0.5 μm adheres, and with the oxidation time, it concentrates on the particles that first adhered to the next 0.3 to 0.5 μm.
This is because soot-like particles having a size of m adhered and overlapped and grew in a lump.

[0023]

[Table 2]

[0024]

As described above, when an oxide film having a thickness of 5 μm or more is grown on the surface of the silicon substrate, the oxidation process is temporarily interrupted, and the etching is removed until the thickness becomes 0.5 μm or less. By growing the oxide film, contamination of the silicon substrate was reduced, and an optical waveguide film having a thickness of 10 μm or more and having excellent characteristics as an optical waveguide was obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an oxidation time and an oxide film thickness when growing an oxide film.

FIG. 2 is a schematic explanatory view showing an outline of growing an oxide film on a substrate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Silicon substrate 2 ... Holder 3 ... Furnace core tube 4 ... Heater 5 ... Cap 6 ... Exhaust pipe

Claims (2)

[Claims] When an oxide film having a thickness of 10 μm or more is formed on a surface of a silicon substrate by a thermal oxidation method, when the oxide film is grown to a thickness of 5 μm or more, the oxidation step is temporarily interrupted, and The grown oxide film is removed by etching with a hydrofluoric acid aqueous solution until the thickness becomes 0.5 μm or less, and an oxide film is grown thereon to form an oxide film having a desired thickness. Manufacturing method of membrane. 2. The method according to claim 1, wherein a part of the oxide film is removed by etching.
2. The method for manufacturing an optical waveguide film according to claim 1, wherein the step of growing an oxide film thereon is performed a plurality of times.