JPH05279015A - Formation of zeolite thin film - Google Patents

Abstract

PURPOSE:To compactly form a zeolite thin film on the surface of a silicon single crystal substrate by forming an oxide film on the substrate surface and then crystallizing zeolite on the oxide film. CONSTITUTION:A reacting soln. is prepared from a starting material contg. an alumina source, a silicon source, an alkali source and water in a specified ratio. A silicon single crystal substrate with the surface coated with an oxide film is dipped in the soln. and hydrothermally treated to crystallize zeolite on the oxide film. The surface of the substrate is coated with the oxide film, for example, by heating the substrate to about 800-1500 deg.C, placing the substrate in a calcining furnace and keeping the substrate in the furnace for about 1-20hr. The thickness of the oxide film is controlled to 0.1-2mum or preferably to 0.5mum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a zeolite thin film on the surface of a silicon substrate used in, for example, humidity sensors and gas sensors.

[0002]

2. Description of the Related Art Conventionally, a humidity sensor using zeolite has a zeolite thin film 32 having a thickness of 1 to 5 μm formed on both sides of a quartz substrate 31 having a thickness of 50 to 1000 μm as shown in FIG. Electrodes 33 are attached to 32 and lead wires 34 are connected. In the humidity sensor 30 having such a structure, the humidity is measured by measuring the capacitance between the electrodes 33.

[0003]

In such a conventional humidity sensor 30, the capacitance C is expressed by the following equation (1) between the electrode spacing d, the electrode area S and the dielectric constant ε. I have a relationship.

C = ε · S / d (1) As is apparent from the equation (1), the sensitivity of the electrostatic capacitance C is S
It greatly contributes to the value of / d, and improves as the electrode area increases and the electrode spacing d decreases.

However, the thickness of the quartz substrate 31 is 50
If it is less than or equal to μm, the mechanical strength is remarkably lowered, and the zeolite thin film 32 is easily broken. Therefore, in the conventional humidity sensor 30, the electrode spacing d cannot be smaller than 50 μm. Therefore, in order to improve the sensitivity of the capacitance C, the electrode area S must be increased.

Since the quartz substrate 31 is non-conductive, the zeolite thin film 32 and the electrode 33 must be formed on both sides of the quartz substrate 31 in order to connect the lead wire 34 to the quartz substrate 31.

For this reason, the conventional humidity sensor 3
At 0, the humidity sensor 30 is maintained while maintaining the sensitivity of the capacitance C.
It is difficult to downsize.

Although the humidity sensor has been described above as an example, a gas sensor using a zeolite thin film for detecting carbon dioxide gas has the same problem.

Therefore, the thickness of the substrate can be further reduced by using a silicon single crystal substrate which is superior in mechanical strength to the quartz substrate 31 and whose electrodes can be easily connected. Further, as shown in FIG. 12, since the lead wire 24 can be directly connected to the conductive silicon single crystal substrate 21 which plays the role of an electrode, it is sufficient to form the zeolite thin film 22 and the electrode 23 on only one surface. As a result, the electrode spacing d'can be made extremely small, and it is conceivable that the sensitivity of the humidity sensor is further improved and the size thereof is further reduced. However, a method for uniformly forming a zeolite thin film on a silicon single crystal substrate has not been known yet.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a novel method for forming a zeolite thin film capable of uniformly forming a zeolite thin film on the surface of a silicon single crystal substrate.

[0011]

According to the present invention, a step of preparing a reaction solution from a starting material containing an alumina source, a silica source, an alkali source and water in a predetermined ratio, and a silicon monolayer coated with an oxide film on the surface thereof. There is provided a method for forming a zeolite thin film, which comprises a step of subjecting a crystal substrate to hydrothermal treatment while being immersed in the reaction solution to crystallize zeolite on the oxide film.

The method for forming the zeolite thin film of the present invention will be described in more detail below. Conventional methods for producing zeolite crystal powder include sodium hydroxide, silica,
It is known that a starting solution such as alumina and water is mixed to prepare a reaction solution, and the reaction solution is subjected to a crystallization treatment in an autoclave to crystallize and precipitate zeolite. Therefore, the present inventors tried to form a zeolite thin film by immersing the silicon single crystal substrate in a reaction solution capable of producing zeolite crystal powder and subjecting it to hydrothermal treatment. However, the dissolution rate of the silicon single crystal substrate by the reaction solution was so high that the silicon single crystal substrate was corroded, and the zeolite thin film could not be formed at all.

In order to eliminate such inconvenience, the present inventors have found that after forming an oxide film on the surface of a silicon single crystal substrate, the zeolite thin film is crystallized on this oxide film. The invention was completed.

The reaction solution used in the method for forming a zeolite thin film of the present invention is prepared by using an alumina source, a silica source, an alkali source and water as starting materials.

When the zeolite thin film to be formed is type A zeolite, the reaction solution is an alumina source, a silica source,
Prepared using an alkali source and water as starting materials.

Here, examples of the alumina source include various aluminum salts such as activated alumina, γ-alumina, alumina hydrate, nitrates and sulfates, and powdered alumina, and sodium aluminate is particularly preferable. .. Further, examples of the silica source include powder silica, colloidal silica, amorphous silica, sodium silicate, silica gel, silicic acid, gel-dissolved silica, water glass and the like, of which water glass is particularly preferable. Also,
Examples of the alkali source include sodium hydroxide and ammonium hydroxide.

First, when an alumina source, a silica source, an alkali source, and water, which are mixed in a predetermined ratio, are mixed with each other, they are gelled to be a solid. Since it cannot be filtered as it is, the gel slurry is prepared by vigorously stirring immediately after mixing these starting materials. In the preparation of the gel slurry, the starting materials may be weighed and then mixed at the same time, or the aqueous solutions of the starting materials may be prepared and then mixed. Further, for example, an aqueous solution obtained by mixing sodium aluminate, sodium hydroxide and water and an aqueous solution obtained by mixing water glass and water can be separately prepared and then the respective aqueous solutions can be mixed.

When a gel slurry is prepared by mixing all the starting materials in this manner, the temperature of the starting materials or their aqueous solution affects the viscosity of the gel slurry obtained. Therefore, it is necessary to adjust the temperature of the starting material or the aqueous solution thereof in advance within the range of 5 to 40 ° C, for example. Particularly preferably, the temperature is adjusted to 32.5 ° C.

The stirring conditions such as the ambient temperature and the stirring time when the starting materials are mixed to prepare the gel slurry also affect the viscosity of the gel slurry. Preferred stirring conditions are an ambient temperature of 24 ° C. and a stirring time of 15 minutes. The process of preparing this gel slurry is called gel aging.

Next, after gel aging, the gel slurry is filtered with a microfilter. Here, the filter diameter of the microfilter changes the amount of minute gel present in the filtrate. The preferred filter diameter is 0.1 to 1μ
m, particularly preferably 0.2 μm.

The resulting filtrate can then be aged to promote nucleation of the zeolite in the filtrate. Preferred aging conditions of the filtrate are 24 ° C. and 1 to 24 hours.

The filtrate may be used as a reaction solution as it is, or may be diluted with deionized water and used as a reaction solution. By diluting with deionized water, the composition and pH of the filtrate can be changed and the state of the zeolite thin film can be controlled. Preferably, the filtrate is diluted about twice with deionized water.

On the other hand, the formed zeolite thin film is ZSM-
In the case of type 5 zeolite, the reaction solution is prepared by mixing an alumina source, a silica source, sodium hydroxide and water in a predetermined ratio.

When the zeolite thin film to be formed is ZSM-5 type zeolite, a silica source, an alumina source, an organic base, sodium hydroxide and water are mixed at an appropriate ratio and a reaction solution having the following composition I is prepared. Prepare.

Composition I Na ₂ O / SiO ₂ 0.05 to 0.35 Al ₂ O ₃ / SiO _{20 to} 0.04 [R] / SiO ₂ 0.05 to 1.5 H ₂ O / SiO ₂ 40 -60 (where [R] is an organic base. The composition ratio is represented by a molar ratio.) Here, the silica source is powder silica, colloidal silica, amorphous silica, sodium silicate. , Silica gel, silicic acid, gel-dissolved silica, water glass and the like. Of these, powdered silica is particularly preferred.

Examples of the alumina source include activated alumina, γ-alumina, alumina hydrate, various aluminum salts such as nitrates and sulfates, powdered alumina and the like. Of these, powdered alumina is particularly preferred.

The organic base mixed as a skeleton-forming agent is
Tetrapropylammonium bromide, tetraalkylammonium bromide (TPABr), propylamine, tetra-n-propylammonium hydroxide (TPA)
OH), tetra-n-propylammonium iodide, and the like, and of these, tetrapropylammonium bromide (TPABr) is particularly preferable.

The oxide film is coated on the surface of the silicon single crystal substrate by, for example, forming a silicon single crystal substrate by 800 to 15
It can be carried out by heating to 00 ° C., putting it in a firing furnace in which oxygen or water vapor is passed, and maintaining it for 1 to 20 hours. The thickness of the oxide film is preferably 0.1 to 2 μm,
0.5 μm is particularly preferred.

On the other hand, formation of the zeolite thin film on the surface of the silicon single crystal substrate is carried out by hydrothermal treatment while the silicon single crystal substrate whose surface is covered with an oxide film is immersed in the above reaction solution. ..

The hydrothermal treatment conditions are, in the case of A-type zeolite, a crystallization time of about 0.5 to 150 hours at a temperature of 20 to 175 ° C. in an autoclave, preferably 80 to 90. Crystallization time of 0.5 to 6 hours at a temperature of ° C. Crystallization time is important in forming a homogeneous zeolite thin film. If the crystallization time is too short, zeolite crystals will not be formed or scattered on the substrate, and if it is too long, the formed zeolite thin film will be exfoliated or transferred to another type of zeolite. The optimum crystallization time depends on the composition of the reaction solution and the treatment temperature. Therefore, it is necessary to select the optimum hydrothermal treatment conditions in accordance with the composition of the reaction solution.

The state of the zeolite thin film formed on the surface of the silicon single crystal substrate differs depending on the method of immersing the substrate in the reaction solution. For example, when the silicon substrate is horizontally immersed in the reaction solution, the state of the zeolite thin film is different between the upper surface side and the lower surface side of the silicon substrate. That is, the zeolite thin film formed on the upper surface side of the silicon substrate is formed in a thin film shape by the zeolite crystals grown in the reaction solution settling and stacking and bonding to each other. For this reason,
The zeolite thin film formed on the upper surface side of the silicon substrate has many gaps and may not be suitable for use as an element of, for example, a humidity sensor or a gas sensor.

On the other hand, in the zeolite thin film formed on the lower surface side of the silicon substrate, only one zeolite crystal directly grown on the oxide film coated on the silicon substrate is formed without gaps. Therefore, the bonded state between the silicon substrate and the zeolite thin film is extremely good, and the film thickness is approximately equal to the size of one crystal (for example, 1 to 4 μm). Thus, the zeolite thin film formed on the lower surface side of the silicon substrate is particularly suitable for use as an element of, for example, a humidity sensor or a gas sensor.

The state of the zeolite thin film formed by vertically immersing the silicon substrate in the reaction solution is almost the same as that of the zeolite thin film formed on the lower surface when horizontally immersed, but slightly. Zeolite crystals grown in the reaction solution fall by gravity and bond to the surface of the zeolite thin film. In order to prevent this, it is effective to place a Teflon shielding plate above the silicon substrate.

The thickness of the zeolite thin film can be controlled by changing, for example, the type of silica source, the composition of the reaction solution, the aging time of the filtrate, the crystallization time, and the like, but it is most controlled by the aging time of the filtrate. preferable.

As described above, by hydrothermally treating the silicon single crystal substrate in the reaction solution, zeolite is coated on the surface of the silicon single crystal substrate to form an oxide film (SiO ₂ ).
The zeolite thin film is formed on the top of the film solidly and densely. Moreover, by coating with an oxide film, the silicon single crystal substrate can be prevented from being corroded by the reaction solution. Here, the zeolite thin film is strongly crystallized on the oxide film because the aluminosilicate molecules in the reaction solution are SiO 2.
Polymerization with ₂ as a binder to form a zeolite skeleton,
This is because the zeolite skeleton further grows and a zeolite thin film is formed on the silicon single crystal substrate, so that the silicon single crystal substrate and the zeolite thin film are bonded at the atomic level.

[0036]

EXAMPLES Examples of the present invention will be described in detail below.

Example 1 Formation of A-type zeolite thin film Here, as starting materials, sodium hydroxide (manufactured by Wako Pure Chemical Industries, Ltd.), water glass (Na ₂ O 17 to 19%, Si)
O ₂ 35 to 38%) (manufactured by Wako Pure Chemical Industries, Ltd.), sodium aluminate _{(Na 2 O30%, Al 2} O 3 35%)
(Kanto Chemical Co., Inc.) was used.

First, 19.4 g of sodium aluminate,
An aqueous solution A was prepared by weighing 8.0 g of sodium hydroxide and 80.1 g of deionized water in a beaker and stirring with a stirrer for 30 minutes.

Next, 26.2 g of water glass and 25.0 g of deionized water were weighed in another beaker and stirred with a stirrer for 30 minutes to prepare an aqueous solution B.

The resulting aqueous solutions A and B were heated to 32.5 ° C. in a constant temperature water bath, then mixed, and stirred vigorously for 15 minutes with a spoon to obtain a gel slurry. The molar composition of the resulting gel slurry was 4.1 Na ₂ O / 2.4SiO ₂ /
It was Al ₂ O ₃ / 104H ₂ O.

Then, the obtained gel slurry is mixed with 0.2
It was filtered through a μm microfilter for 30 minutes. The obtained filtrate was aged at 24 ° C. for 1 hour and then diluted 2-fold with deionized water to obtain a reaction solution.

The obtained reaction solution 12 is housed in a Teflon container 11 as shown in FIG. 1, and the reaction solution 12 is covered with an oxide film having a thickness of 0.4 μm.
3 was immersed substantially horizontally while holding both ends thereof by the support base 14.

The Teflon container 11 was placed in an autoclave, and the silicon single crystal substrate 13 was hydrothermally treated under the conditions of a temperature of 82.5 ° C. and a crystallization time of 4 hours. Then, the silicon single crystal substrate 13 was washed and dried.

The zeolite thin films thus formed on the upper and lower surfaces of the silicon single crystal substrate were observed by an electron microscope and subjected to X-ray diffraction. The structures of the zeolite crystals in the zeolite thin films formed on the upper surface side and the lower surface side of the silicon single crystal substrate are shown in the electron microscope photographs of FIGS. 2 and 3, respectively. 2 (A),
3B shows the surface structure and cross-sectional structure of the zeolite crystal on the surface on the upper surface side, and FIGS. 3A and 3B show the surface structure and cross-sectional structure of the zeolite crystal on the surface on the lower surface side.

From these results, it was confirmed that the A-type zeolite was crystallized on both the upper surface side and the lower surface side of the silicon single crystal substrate. Further, the zeolite thin film formed on the surface of the oxide film on the upper surface side of the silicon single crystal substrate has many gaps and is not suitable for use as an element of a humidity sensor or a gas sensor. On the other hand, the zeolite thin films formed on the lower oxide film surface are formed without gaps between each other,
It was confirmed to be suitable for use as an element of a zeolite sensor.

Example 2 A zeolite thin film was formed on a silicon single crystal substrate according to the same procedure as in Example 1 except that the aging time of the filtrate was changed to 1 hour and 24 hours. The cross-sectional structure of the zeolite crystal in the zeolite thin film formed on the lower surface side of the silicon single crystal substrate is shown in the electron micrographs of FIGS. 4 and 5.

From this result, it was confirmed that the thickness of the zeolite thin film formed on the surface of the substrate can be controlled by changing the aging time of the filtrate.

Example 3 A zeolite thin film was formed on a silicon single crystal substrate in the same procedure as in Example 1 except that the crystallization time was changed to 40 minutes, 1, 2, 4, 8 hours. The surface structure of the zeolite crystals in the zeolite thin film formed on the lower surface side of the silicon single crystal substrate is shown in electron micrographs of FIGS.

As is clear from FIGS. 6 to 11, it was confirmed that the state of the zeolite thin film formed on the silicon single crystal substrate was greatly influenced by changing the crystallization time.

Example 4 Formation of ZSM-5 type zeolite thin film As starting materials, sodium hydroxide (manufactured by Wako Pure Chemical Industries, Ltd.), tetrapropylammonium bromide (T
PABr; Wako Pure Chemical Industries, Ltd.) and silicon dioxide (Kanto Chemical Co., Inc.) were used.

First, the above-mentioned starting materials are mixed at a predetermined ratio and placed in a Teflon container. The mixture was aged at 24 ° C. for 24 hours while stirring with a shaker to prepare a reaction solution having the following composition.

0.050 Na ₂ 0 / SiO ₂ /0.668TPABr/49H ₂ O The obtained reaction solution was placed in a Teflon container 11 as in Example 1, and the thickness of the reaction solution was 0.4 μm. The silicon single crystal substrate 13 coated with the oxide film of No. 1 was immersed substantially horizontally while holding both ends of the silicon single crystal substrate 13 with the support 14.

The Teflon container 11 was placed in an autoclave, and the silicon single crystal substrate 13 was subjected to hydrothermal treatment under the conditions of a temperature of 120 ° C. and a crystallization time of 24 hours. After that, the silicon single crystal substrate 13 is washed and dried, and ZS is formed on the surface.
A silicon single crystal substrate on which an M-5 type zeolite thin film was formed was obtained.

On the surface which was the lower surface side when the hydrothermal treatment of the obtained silicon single crystal substrate was performed, a uniform ZSM-5 type zeolite thin film having a thickness of 1.5 μm was firmly attached to the silicon single crystal substrate. It was confirmed that it was formed by binding to.

[0055]

According to the method for forming a zeolite thin film of the present invention, a zeolite thin film can be easily formed on a silicon single crystal substrate in a required state. As a result, it greatly contributes to the performance improvement and miniaturization of the humidity sensor and the gas sensor using the zeolite thin film.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a hydrothermal treatment step of the method for forming a zeolite thin film of the present invention.

2A is an electron micrograph showing the surface structure of zeolite crystals in a zeolite thin film formed on the upper surface side of a silicon single crystal substrate according to the method for forming a zeolite thin film of the present invention, and FIG. 2B is the same. An electron micrograph showing a cross-sectional structure of a zeolite crystal.

FIG. 3 (A) is an electron micrograph showing the surface structure of zeolite crystals in a zeolite thin film formed on the lower surface side of a silicon single crystal substrate according to the method for forming a zeolite thin film of the present invention, and (B) is the same. An electron micrograph showing a cross-sectional structure of a zeolite crystal.

FIG. 4 shows a method for forming a zeolite thin film according to the present invention,
3 is an electron micrograph showing a cross-sectional structure of zeolite crystals in the zeolite thin film formed on the lower surface side of the silicon single crystal substrate when the filtrate was aged for 1 hour.

FIG. 5 shows a method of forming a zeolite thin film according to the present invention.
An electron micrograph showing a cross-sectional structure of zeolite crystals in a zeolite thin film formed on the lower surface side of a silicon single crystal substrate when the filtrate was aged for 24 hours.

FIG. 6 shows a method for forming a zeolite thin film according to the present invention.
The electron micrograph which shows the surface structure of the zeolite crystal in the zeolite thin film formed in the lower surface side of the silicon single crystal substrate when crystallization time is 40 minutes.

FIG. 7 shows a method for forming a zeolite thin film according to the present invention.
The electron micrograph which shows the surface structure of the zeolite crystal in the zeolite thin film formed in the lower surface side of the silicon single crystal substrate when crystallization time is 1 hour.

FIG. 8 shows a method for forming a zeolite thin film according to the present invention.
The electron micrograph which shows the surface structure of the zeolite crystal in the zeolite thin film formed in the lower surface side of the silicon single crystal substrate when crystallization time is 2 hours.

FIG. 9 shows a method for forming a zeolite thin film according to the present invention,
The electron microscope photograph which shows the surface structure of the zeolite crystal in the zeolite thin film formed in the lower surface side of the silicon single crystal substrate when crystallization time is 4 hours.

FIG. 10 is an electron micrograph showing the surface structure of zeolite crystals in the zeolite thin film formed on the lower surface side of the silicon single crystal substrate when the crystallization time is 8 hours according to the method for forming a zeolite thin film of the present invention.

FIG. 11 is an electron micrograph showing the surface structure of zeolite crystals in the zeolite thin film formed on the lower surface side of the silicon single crystal substrate when the crystallization time is 8 hours according to the method for forming a zeolite thin film of the present invention.

FIG. 12 is a sectional view showing an example of a zeolite humidity sensor using a silicon single crystal substrate.

FIG. 13 is a sectional view showing a zeolite humidity sensor using a conventional quartz substrate.

[Explanation of symbols]

11 ... Teflon container, 12 ... Reaction solution, 13 ... Silicon single crystal substrate, 14 ... Support stand.

Claims (1)

[Claims] 1. A step of preparing a reaction solution from a starting material containing an alumina source, a silica source, an alkali source and water at a predetermined ratio, and a step of adding the reaction solution to a silicon single crystal substrate whose surface is coated with an oxide film. A method for forming a zeolite thin film, comprising a step of subjecting a zeolite to a hydrothermal treatment in the immersed state to crystallize the zeolite on the oxide film.