JP2001185786A - Planar type light amplifying element using aerosol process and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive planar type light amplifying element with high functionality in which erbium is uniformly doped by using an aerosol process and a method for manufacturing this light amplifying element. SOLUTION: An under-clad layer 20 is adhered on a silicon substrate 10, and a core layer is adhered on the under-clad layer 20 through an aerosol process, and the core layer is etched so that a core 30 can be prepared, and an over-clad layer 40 is formed on the under-clad layer 20 and the core 30 through the aerosol process. In this case, a large amount of P2O5 and a small amount of Na2O is doped to the core, and the covalent bond of SiO2 is disconnected so that a large amount of erbium can be doped to the core without generating clustering.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a planar optical amplifier manufactured using an aerosol process and a method of manufacturing the same, and more particularly, to a planar optical amplifier doped with erbium (Er).

[0002]

2. Description of the Related Art In optical communication, most transmission lines use optical fibers. However, there are technical limitations in fabricating multi-channel devices such as multi-segment optical couplers and wavelength division multiplexer (WDM) devices using optical fibers.

At present, 1.3 μm and 1.5 μm wavelength light sources are used as light sources for optical communication. Among them, 1.5 μm
Is the lowest loss in silica and is widely used as an optical communication wavelength. However, as the optical transmission distance has gradually increased, the optical transmission method using optical fibers has begun to have limitations.

[0004] Rare earth (eg, erbium) doped fiber amplifiers (EDFAs) exhibit ideal properties that can overcome optical fiber losses in the 1.55 μm wavelength band. Accordingly, such an optical amplifier is mainly used in spite of an increase in the optical transmission distance, and its application range is widened by being used as various optical passive elements.
However, the manufacturing process of the EDFA is very strict, and only a very small amount (several ppm / m) of erbium can be added to the optical amplifier with the current technology. When only a small amount of erbium is added, the length of the amplifier becomes longer (for example, in the case of an amplification factor of 200, 20 to 5 to obtain a gain of 20 dB).
0 m length is required), and the manufacturing cost is high.

In addition, when a large amount of erbium is added, each erbium ion cannot cut the structure of the silica having a covalent bond, and erbium is placed at an inappropriate position. And form a cluster. Such a clustering phenomenon causes light scattering to cause light loss and deteriorates amplification characteristics. To remove such clustering,
Erbium ions are easily located by adding alkali ions into the silica to break the covalent bond structure and convert the silica structure to an ionic bond structure. However, in the case of the conventional optical fiber manufacturing method, it is known that it is very difficult to add alkali ions into silica, and it is known that only a very small amount is added, and it is reported that alkali and erbium ions cannot be uniformly distributed. Have been.

Therefore, instead of the conventional optical fiber manufacturing method, there is a need for a technology capable of overcoming the limit of the transmission length by integrating optical fibers. For this reason, planar lightwave circuits (PLCs) have been developed that implement optical signal paths on a planar substrate. By using such a PLC technique, it is possible to manufacture a small and low-priced optical amplifier. Also, active and passive devices can be integrated on a single substrate, and various integrated optical modules can be realized. Erbium doping can also be applied to a planar optical device.

[0007] Using such a PLC technique, a planar optical device can be manufactured from various materials such as semiconductors, silica, and polymers. In the case of silica, transmission loss is low and semiconductors such as photolithography and etching are used. Since it can be processed by applying a process technique, it is used as a main material of a planar optical device.

In order to form a silica film on a substrate, it is necessary to use a flame hydrolysis deposition method (FH).
D), a chemical vapor deposition method (CVD), a sol-gel method, a powder method, and the like. Among these techniques, the FHD method has a higher deposition rate and is easier to form a thick film than other techniques, and is widely used in the manufacture of planar optical devices. By using high-purity silica, the cross section of the planar optical waveguide can be manufactured similarly to the cross section of the optical fiber. By the FHD method, an oxide such as B, P, or Ge is added to the optical element as a dopant for controlling the refractive index and the thermal process temperature of the silica optical waveguide.
Conventional methods of manufacturing optical waveguides using the FHD method are disclosed in European Patent Publication Nos. 0,545,432 and 0,75.
No. 1,408.

[0009]

However, the FHD method has a number of disadvantages. First, there is a high risk because chloride materials with high vapor pressure are used. Second, since the temperature of the flame used for depositing the waveguide film is as high as 1500 ° C. or more, it is difficult to add a large amount of dopant. Third, it is difficult to adjust the composition ratio of the dopant. For this reason, when forming an under cladding layer, a core, and an over cladding layer using the FHD method, it is difficult to control the refractive index and the thermal process temperature of each layer. Above all,
In the case of forming the over cladding layer using the FHD method, it is difficult to lower the temperature of the thermal process to 1100 ° C. or less because there is a limit in the material composition. If the temperature of the thermal process for the overcladding layer is too high, it can adversely affect the shape of the core formed below the overcladding layer and cause serious problems in optical signal processing.

Further, when the FHD method is used, it is not possible to add a rare earth such as an alkali ion or an alkaline earth having a low vapor pressure to the light propagation layer, so that there is a disadvantage that the clustering phenomenon of erbium cannot be prevented.

Therefore, in the conventional FHD method, as described above, the composition of the material is limited, and it is very difficult to manufacture a film in which erbium is added at a uniform high concentration.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a low-cost, high-performance planar optical amplifier element which is uniformly doped with erbium using an aerosol process.

It is another object of the present invention to provide a method for manufacturing a planar optical amplifier having a core and an over clad generated by using an aerosol process.

It is still another object of the present invention to provide a method of manufacturing a planar optical amplifier device in which the content of a dopant can be easily adjusted by lowering a thermal process temperature and the shape of a core is not changed.

Still another object of the present invention is to provide a method of manufacturing a planar type optical amplifier which can easily add a metal having a low vapor pressure to the optical amplifier.

It is still another object of the present invention to provide a short, long and low-priced planar optical amplifier by realizing a high erbium density.

[0017]

According to a preferred embodiment of the present invention, there is provided a method for manufacturing a planar optical amplifier, comprising the steps of: depositing an under-cladding layer on a silicon substrate; A second step of applying a core layer on the undercladding layer using an aerosol process, and a third step of etching the core layer to form a core.
And a fourth step of forming an over-cladding layer on the under-cladding layer and the core using an aerosol process.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a sectional view of a planar optical amplifier according to the present invention. The planar optical amplifier of the present invention comprises a silicon substrate 10, an under cladding layer 20, a core 30 formed by an aerosol process, and an over cladding layer 40.

Hereinafter, a planar type optical amplifier according to the present invention will be described.
Will be described in detail. First, the usual silica film formation
The under cladding layer 20 using a silicon-based method.
Formed on plate 10. The under cladding layer 20 can be various
F that can easily produce films of various thicknesses and can be mass-produced
Using the HD method, for example, SiO 2_Two, B_TwoO_ThreeAnd P_TwoO _Five
Can be formed.

After the formation of the under cladding layer 20, a core layer as a borosilicate glass-based film to which alkali ions and a large amount of erbium are added is formed on the under cladding layer 20 by an aerosol process. More specifically, after the fine particles are deposited on the under clad through an aerosol process, the fine particles are melted through a thermal process (densification process) to form a glass film, thereby forming a core layer. After the formation of the core layer, the core layer is etched using a semiconductor process technology to form an optical circuit pattern.
(Core 30 in FIG. 1) is left. According to the present invention, Na ₂
By adding appropriate amounts of dopants such as O and P ₂ O _{5 to} the core layer, large amounts of erbium can be added to the core layer without causing clustering.

After the formation of the core 30, the overcladding layer 40 is formed on the undercladding layer 20 and the core 30 by an aerosol process. According to the present invention, when forming the over cladding layer 40, Na ₂ O is converted to SiO ₂ -B ₂
The melting temperature is lowered by adding to the O ₃ -based glass. Through such a method, the temperature of the thermal process is 1
The temperature can be lowered to 100 ° C. or less. As a result, even after a thermal process is applied to the over cladding layer 40, the shape of the core 30 is maintained as it is.
An optical element with high reproducibility can be obtained.

As described above, the aerosol process used in the present invention is a process in which a liquid sol is formed into particles through an ultrasonic oscillator, and the particles are oxidized by an oxygen-hydrogen flame and deposited on a substrate. Through such an aerosol process, the amount of B, P, and Ge can be freely adjusted by limiting the addition of dopants much less than the conventional FHD method. Experiments have shown that B ₂ O ₃ , P ₂ O ₅
And GeO ₂ are added in an amount of 0.1 to 60 mol% and 0.1, respectively.
Amorphous clear glass films of 1 to 45 mol% and 0.1 to 20 mol% could be obtained.

According to the aerosol process, a large amount of metal ions such as alkali metals and rare earths having a low vapor pressure can be added. Therefore, the aerosol process has the following advantages. 1. It is possible to form a multi-supported glass film having a small optical transmission loss. 2. The deposition rate is high, and it is easy to form a thick film of 1 to 100 μm. 3. Since it has almost the same size and refractive index as the optical fiber, the connection loss with the optical fiber is small. 4. Economical because mass production is possible at room temperature.

Hereinafter, an exemplary method of manufacturing an optical amplifying device using an aerosol process will be described in detail.

First, tetraethyl orthosilicate (TEO)
S), B [(CH_Three)_TwoCHO]_Three, H_ThreePO _Four, Ge (0C_TwoH_Five)
_Four, Er (NO_Three) ・ 5H_TwoO₇, HNO_Three, Methanol, steam
A sol solution is made using a material such as stored water. others
Therefore, the mixing ratio of distilled water and methanol solvent for TEOS
Hydrolyze for 1 hour in a solution that is 5: 1. Addition
Nitric acid is used as a catalyst for water splitting. phosphoric acid
If the amount of nitric acid is large, the amount of nitric acid needs to be reduced to a minimal amount
is there. In such a solution, B [(CH_Three)_TwoCHO]_Threeas well as
Ge (OC_TwoH_Five)_FourSlowly, and stir with a magnetic stirrer.
React for 3 hours with stirring. This set of sol solutions
The composition ratio is determined by the composition ratio of the glass film to be formed.
It is. Preferably, according to the present invention, a planar optical device
Of the sol solution used in the formation of each layer in the water
The ratio is as shown in [Table 1] below.

[0027]

[Table 1]

In the case of a sol solution for the core layer, preferably, SiO ₂ , B ₂ O ₃ , P ₂ O ₅ , Na ₂ O and Er ₂ O ₃
The content of each is 40 to 50 mol%, about 5 mol%, 45 to 5 mol%.
5 mol%, 2-3 wt% and 1-2 wt%. When the difference in refractive index between the core layer and the under cladding layer is to be set to 0.3Δ, GeO ₂ of ₂ to 3 wt% is added to the sol solution for forming the core layer to reduce the difference in refractive index. If 0.75Δ is to be used, add 10 to the sol solution.
Adding ～11Wt% of GeO _2.

More preferably, in the case of the over cladding layer, the content of SiO ₂ , B ₂ O ₃ and Na ₂ O is _{20 to 3} respectively.
0 mol%, 70-80 mol% and 2-3 wt%.

Next, the sol solution is put into a container to which an ultrasonic oscillator is attached, and an aerosol is produced. When making an aerosol using ultrasonic waves, if the ultrasonic waves pass through the liquid and reach the interface between the liquid and the gas, droplets are generated at the interface. At this time, the size of the droplet depends on the density and surface tension of the liquid and the angular frequency of the ultrasonic wave. Therefore,
By changing the density and surface tension of the liquid, the size of the aerosol can be adjusted. According to a preferred embodiment of the present invention, 1.5 MHz ultrasound is used, so that the size of the aerosol has a diameter of less than 4 μm.

FIG. 2 is a schematic diagram schematically showing an aerosol process apparatus for manufacturing a planar optical amplifier according to the present invention.

The aerosol produced by the method described above is applied to the undercladding layer 20 on the silicon wafer (when forming the core layer) using the torch 70 in FIG. It is deposited on the undercladding layer 20 and the core 30 (during the formation of the cladding layer 40). Aerosol torch 7 with argon gas
0 and deposited on a silicon wafer 60 maintained in a wafer holder 50 while being oxidized by an oxyhydrogen flame. At this time, the temperature of the wafer holder 50 is maintained at 200 ° C. by using the temperature controller 110 to minimize the vaporization of B ₂ O ₃ and P ₂ O ₅ . In addition, in order to form a film having a uniform thickness, the torch 70 deposits the oxide powder while moving in the x and y directions on the XY stage 90 under the control of the computer 100.

After deposition, the silicon wafer 60 is subjected to a thermal process. First, a silicon wafer 60 is removed to remove water of crystallization or organic matter from the applied powder.
Heat treatment is performed in an oxygen atmosphere at 00 ° C. for one hour. next,
The film is dried and heat treated in a furnace for 2 hours to melt the oxidized powder. At this time, the melting temperature varies depending on the composition of the film. According to the invention, the melting temperature for the core layer is 125
0 to 1330 ° C, and the melting temperature for the over cladding layer 40 is 900 to 1000 ° C. After the melting process, the silicon wafer 60 is quenched in air to produce a glass film.

FIGS. 3 to 6 are graphs showing characteristics of a glass film manufactured by using an aerosol process. FIGS. 3 and 4 show (92-x) SiO ₂ -xB ₂ O ₃ -8P. ₂ O ₅
It is a graph showing the characteristics of a glass film having a composition ratio of
5 and 6 upon addition of GeO ₂ and Er ₂ O ₃ in the composition of _{62SiO 2 -30B 2 O 3 -8P 2} O 5, is a graph showing a change in the refractive index.

More specifically, FIG. 3 shows that (92-x) SiO_Two
-XB_TwoO_Three-8P_TwoO_FiveB in a solution or membrane of_TwoO_Threeof
Relationship between refractive index and melting temperature of glass films with respect to concentration
FIG. B_TwoO_ThreeContent of 20 to 70 mol
%, The refractive index becomes 1.4683 from 1.4589.
To increase. Generally, SiO_Two-B_TwoO_ThreeAt B_TwoO _Three
When the content increases, the refractive index decreases and then increases, but the
When cooling after the melting process, as shown in FIG.
Results are obtained. B_TwoO_ThreeWhen the content of_TwoO_Three
Changes from a triangular structure to a pyramid structure. Put Si
In other words, the glass structure will be more compact
Therefore, the refractive index increases.

The refractive index of glass depends on the cooling rate and the conditions of the thermal process. On the other hand, in manufacturing the optical amplifying device, the under cladding layer 20 is further subjected to the thermal process applied when the core 30 and the over cladding layer 40 are formed, that is, undergoes two thermal processes. . Similarly, the thermal process for the overcladding layer 40 causes the core 30 to go through one additional thermal process.

More specifically, the under cladding layer 20 has a thickness of 1
Made into glass by a 350 ° C. thermal process. Next, a core layer is deposited on the under cladding layer 20.
This core layer is produced by a 1320 ° C. thermal process. Thus, the under cladding layer 20 goes through two thermal processes. After etching the core layer to produce the core 30, an overcladding layer 40 is deposited over the undercladding 20 and the core 30 and at 1000 ° C.
The glass is produced through the thermal process. In this case, the temperature of the under cladding layer 20 and the core 30 is 1000 ° C.
Through a thermal process.

Accordingly, the undercladding layer 20 and the core 30 each have a structure other than the thermal process applied during the formation.
Because of the additional thermal process described above, the refractive index of the undercladding layer 20 and the core 30 may be undesirably changed. Therefore, it is important to consider the effect of the refractive index of the previously created layer due to the thermal process.

FIG. 4 illustrates the effect of such a thermal process.
To understand, the (92-x) SiO _Two-XB_TwoO_Three-8P_Two
O_FiveAfter obtaining the glass of the composition,
Bake samples above glass transition temperature for 24 hours
If an annealing experiment is performed, the temperature depends on the annealing temperature.
6 is a graph showing a change in the refractive index of a lath film. Glass transition
The refractive index rises sharply near the transfer temperature, but if the
Note that there is almost no change in refractive index at annealing temperature.
I want to be watched. That is, the previously formed underclad
In order not to affect the refractive index of the layer 20, the core 30 and the
The glass forming temperature of the over cladding layer 40 is 900 ° C.
You have to do more.

When the rate of change of the refractive index between the undercladding layer 20 and the core 30 is referred to as Δ%, these two layers are 20 mol% and 70 mol%, respectively (ie, x = 20 mol%).
If and x = 70) B produced by the ₂ O _3, the change rate delta% of the refractive index can be a maximum 0.7. However, in order to manufacture various optical devices, it is necessary to adjust the refractive index more easily.

FIG. 5 shows a graph of 62SiO ₂ -30B ₂ O ₃ -8P ₂
The O ₅ is a graph showing the refractive index of the case of adding GeO ₂ of xwt% (x = 2~12). Referring to FIG.
When eO ₂ is increased from 2% to 12%, the refractive index becomes 1.4.
It turns out that it increased to 1.4716 from 633. By adding GeO ₂ in this way, 0.3 to 0.3 is obtained.
75 different refractive index change rates Δ% are obtained.

FIG. 6 is a graph showing a change in the refractive index when Er ₂ O ₃ is added. Note that the higher the Er ₂ O ₃ content, the higher the refractive index. Therefore, in order to obtain a desired refractive index, B ₂ O ₃ , P ₂ O ₅ , GeO ₂ , Na ₂
The content of O and Er ₂ O ₃ should be adjusted appropriately to.

According to a preferred embodiment of the present invention, the thermal processing temperature, the composition ratio (mol%) of the formed glass film and the refractive index of the formed glass film for each layer are as follows.

[0044]

[Table 2]

In Table 2, the composition ratio of the formed glass film is measured by electron probe microanalysis (EPMA). As described with reference to FIG. 4, the thermal process temperature of the over cladding layer 40 was set to 900 ° C. or higher so that the refractive index of the core did not change. In addition, the thermal processing temperature of the over cladding layer 40 was set to 1000 ° C. or less so that the shape of the core 30 was not deformed. In the same composition ratio range as in [Table 2], the refractive index change rate between the under cladding and the core is 0.3 to 1.0 Δ%.
Value was obtained. More preferably, the temperature of the thermal process for the undercladding, core and overcladding is 1350 ° C, 1300 ° C and 900 ° C to 1 ° C, respectively.
000 ° C.

In the method of manufacturing an optical amplifier according to the present invention, the embodiment in which erbium is added has been described.
A rare earth material such as tellurium may be added.

While the preferred embodiment of the present invention has been described above, those skilled in the art will be able to make various modifications without departing from the scope of the present invention.

[0048]

Therefore, according to the present invention, a large amount (20 to
By cutting the covalent bond of SiO ₂ to P ₂ O ₅ and a small amount of Na ₂ O of 45 mol%) was added to the core, without causing clustering, it is possible to add a large amount of erbium in the core.

Further, according to the present invention, a large amount of erbium can be added to the core, so that the length of the optical amplifying element can be shortened and a low-cost optical amplifying element can be manufactured.

According to the present invention, by adding Na ₂ O to the overcladding, the temperature of the thermal process of the overcladding is reduced to a temperature that does not affect the core, that is, from 900 ° C. to 1000 ° C. Can be lowered.

Further, according to the present invention, by using a material capable of applying a thermal process to the over cladding at a temperature of at least 100 ° C. or lower than the melting temperature of the core, the range of the melting temperature of the core layer can be increased. Since it can be further expanded, an optical element with various functions can be manufactured. Furthermore, since the core is not deformed during the thermal process of the over cladding, an optical element with high reproducibility during manufacturing can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a planar optical amplifier according to the present invention.

FIG. 2 is a schematic view schematically showing an aerosol process apparatus for manufacturing a planar optical amplifier according to the present invention.

It is a graph showing the relationship between [3] _{(92-x) SiO 2 -xB} 2 O 3 -8P 2 O 5 the refractive index of the glass film and the melting temperature.

FIG. 4 is a graph showing a change in a refractive index of a glass film depending on an annealing temperature.

FIG. 5 is a graph showing a change in the refractive index when GeO ₂ is added.

FIG. 6 is a graph showing a change in the refractive index when Er ₂ O ₃ is added.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Silicon substrate 20 Under cladding layer 30 Core 40 Over cladding layer 50 Wafer holder 60 Silicon wafer 70 Torch 80 Aerosol generator 90 XY stage 100 Computer 110 Temperature controller

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Choi Eun Gwangju Gwangyuk-Sook Buk-Moonhung 2 Dong 787-6 Woseong-A, Part 101 Dong 609 (72) Inventor Mun Jong-ha Gwangju Kwang-Yeok, Korea Gyeong Apartment 104 Dong 1103

Claims (18)

[Claims] 1. A planar optical amplifier, comprising: a silicon substrate; an undercladding layer formed on the silicon substrate; a core adhered on the undercladding layer by an aerosol process; A planar type optical amplifying device comprising: the under cladding layer and an over cladding layer provided on the core. 2. The planar optical amplifier according to claim 1, wherein a difference in refractive index at an interface between the under cladding layer and the core is 0.3 to 1.0 Δ%. . 3. The method according to claim 1, wherein the core comprises 0.1 to 5.0 mol% of Er.
_2. The planar optical amplifying device according to claim 1, further comprising ₂ O ₃ . 4. The method according to claim 1, wherein the core comprises 50 to 80 mol% of SiO.
_2, 0.5 to 1 mol% of B ₂ O _3, 20 to 45 mol% of P ₂
Planar optical amplifying element according to claim 1, characterized in that it contains O ₅ and 1 to 10 mol% of GeO _2. 5. The method according to claim 1, wherein the core comprises 0.1 to 5.0 mol% of Na.
_2. The planar optical amplifying element according to claim 1, comprising ₂ O. 6. The method according to claim 1, wherein the over cladding layer has a thickness of 40 to 70.
Planar optical amplifying element according to claim 1, characterized in that it contains SiO ₂ and 25-55 mole% B ₂ O ₃ in mole%. 7. The over-cladding layer according to claim 7, wherein the over-cladding layer has a thickness of 0.1 to 5.
_2. The planar optical amplifier according to claim 1, containing 0 mol% of Na2O. 8. A method for manufacturing a planar optical amplifier, comprising: a first step of applying an under cladding layer on a silicon substrate; and applying a core layer on the under cladding layer using an aerosol process. A second step, a third step of forming a core by etching the core layer, and a fourth step of forming an over cladding layer on the under cladding layer and the core using an aerosol process. A method for manufacturing a planar type optical amplifying element. 9. A thermal process comprising: forming a sol solution; forming an aerosol using the sol solution; depositing the aerosol on the undercladding layer; The method of claim 8, further comprising: melting the applied aerosol through cooling; and cooling the molten aerosol to form a glass film. 10. The method according to claim 1, wherein the thermal process comprises:
The method according to claim 9, wherein the method is performed at a temperature in a range of 30 ° C. 10. 11. The sol solution comprises 0.2 to 5.0 wt.
10. The method of claim 9, wherein the method further comprises Er ₂ O ₃ . 12. The sol solution contains 30 to 70 mol%.
10. Planar mold according to claim 9, characterized in that it contains: SiO ₂ , 5 to 10 mol% B ₂ O ₃ , 25 to 65 mol% P ₂ O ₅ and 2 to 12 mol% GeO _2. Optical amplifying element manufacturing method. 13. The method according to claim 1, wherein the sol solution is 40 to 50 mol%.
SiO _2, from about 5 mole% of B ₂ O _3, 45~55 mol% of P ₂ O _5, and 2-3 mol% or 10-11 mol% of Ge
The method of claim 9, wherein O ₂ is contained. 14. The planar optical amplifier according to claim 9, wherein said sol solution contains 0.5 to 5.0 mol% of Na ₂ O. 15. The method according to claim 15, wherein the fourth step comprises: generating a sol solution; forming an aerosol using the sol solution; and depositing the aerosol on the core and the undercladding layer. 9. The planar optical amplifier according to claim 8, comprising: melting the applied aerosol through a thermal process; and cooling the melted aerosol to form a glass film. Production method. 16. The method of claim 1, wherein the melting step is performed at 900 to 1000 ° C.
The method of claim 15, further comprising performing a thermal process at a temperature in a range. 17. The method according to claim 17, wherein the sol solution is 20 to 50 mol%.
The method of claim 15, wherein the method further comprises: SiO ₂ and 50 to 80 mol% of B ₂ O ₃ . 18. The method according to claim 18, wherein the sol solution is 0.5 to 5.0 wt.
20. The method of claim 17, further comprising the addition of about ₂₀ % by weight of Na2O.