JPH01207654A - Manufacture of gas sensor - Google Patents

Abstract

PURPOSE:To use a mask of photoresist, etc., at the time of thin film manufacture by exposing a gas for raw material to plasma by RF discharging, etc., which is triggered by an excitation source and forming a metal oxide thin film. CONSTITUTION:The raw material gas for the metal oxide thin film is admitted to a reaction chamber 2 from a gas intake 20. Then, microwave plasma is produced in a container 26 with a microwave supplied from a microwave power source 24 and put in circular motion by applying a magnetic field to the container 26 from an electromagnet 30 to suppress the contact of the plasma with the container wall while making the plasma flow in the reaction chamber 2 through a grid 34. Positive ions in the plasma are accelerated through the operation of the grid 34, etc., to enter the reaction chamber 2 and when it reaches the part between discharging electrodes 10 and 12, the RF discharging is triggered with the ions. Consequently, the discharging is started even under low pressure and even if the electric power of a RF power source is low to decompose the raw material gas, thereby forming the metal oxide thin film on a substrate 18. Further, the mask of the photoresist is formed on the substrate 18 and removed after the thin film is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method of manufacturing a gas sensor using plasma discharge.

[Prior Art] A technique for forming a thin film using RF discharge (plasma emission 711 using radio waves) is well known. In this case, the total pressure of the raw material gas is often about 0.1 Torr.

When the pressure of the source gas is lowered below this level, discharge becomes less likely to occur. When a discharge is caused at a high discharge voltage, that is, in a low vacuum, the surface temperature of the substrate increases significantly. Therefore, a photoresist mask cannot be used, and the fine processing of the thin film becomes conical.

The invention was made by injecting microwave plasma, electron beams, or ion beams into a discharge source such as Ili', and triggering an RF discharge using these ions or electrons to start a discharge at a lower voltage. I found out. The inventor then considered manufacturing a gas sensor using this method.

[Problem of the Invention] An object of the present invention is to enable the use of a mask such as a photoresist during thin film production.

[Structure and operation of the invention] In this invention, gold! +1 Exposing the oxide raw material gas to the metal oxide source 112
Form the mouth. This discharge is performed by rigging with another excitation source.

Examples of gold II4 oxide thin films include metal oxide semiconductors such as 5nOt and ZnO, proton conductors such as hydrated compounds of 5b2O, and oxygen ion conductors such as Zr02.

For example, microwave plasma is used as the excitation source. Preferably, a magnetic field is applied to the microwave plasma generation section. When a magnetic field is applied, the motion of electrons and ions in the plasma on a plane perpendicular to the magnetic field becomes a rotational motion, which prevents dissipation of the plasma due to collisions between these charged particles and the vessel wall. Some of the electrons in the plasma rotate in synchronization with the microwave and are efficiently accelerated by the microwave's electric field.

This is a resonance between microwaves and electrons, and this resonance is used to absorb microwave energy into plasma extremely efficiently.

It is preferable to provide a grid between the microwave plasma generation section and the RF or other discharge section.

When the potential of the grid is controlled to be negative with respect to the two plasmas, the electrons in the microwave plasma are repelled by the grid and pushed back into the microwave plasma. Further, the potential of the microwave plasma generation section is preferably positive with respect to the potential of the plasma generation section such as RF in order to irradiate only the positive ions in the plasma to the plasma generation section such as the IP.

As other excitation sources, any one of electron beams, ion beams, etc. can be used. However, when an electron beam is used, the filament of the beam source is oxidized by oxygen in the source gas, so there is a drawback that oxygen cannot be added to the source gas. Therefore, microwaves or ion beams are preferable.

As the raw material gas for the metal oxide thin film, for example, a mixed gas of a low boiling point metal compound and oxygen may be used.

The mixed gas may include a gas such as Ar that is easy to discharge and has low ionization energy. Type of metal oxide S
Now, as a low boiling point compound of Sn, 5nC1
+, Sn(CHs)-, etc. Of these, Sn(CHy)-, which has no corrosive properties, is preferred. Of course, the metal oxide thin film may be manufactured by first forming the metal thin film and then oxidizing it.

The characteristics of this manufacturing method are as follows. First, the upper temperature limit of the substrate on which the thin film is to be formed is small.

For this reason, it becomes possible to use a photoresist, and it becomes possible to form a fine thin film using a mask pattern using a photoresist. That is, it becomes possible to manufacture small gas sensors.

Second, it is easy to form a locally uniform thin film. Therefore, a large number of sensors can be produced simultaneously using a large substrate. Moreover, the variation in the obtained sensor is small. For example, as an example of the conventional method, when a solution of an organic compound of Sn is sprayed onto a heated substrate and thermally decomposed on the substrate, the resistance value of the thin film varies greatly depending on the position and angle from the spray.

Third, amorphous thin films can be easily obtained. Accordingly, a gas sensor with different characteristics from the conventional method can be obtained.

[Example] Structure of currency a FIG. 1 shows the structure of the thin film manufacturing apparatus used.

This device was developed by one of the inventors and is new in itself. In the figure, 2 is a reaction chamber, 4 is a diffusion pump, 6 is a rotary pump, and 8 is an exhaust port from the reaction chamber 2. 10 is! (The discharge electrode for F discharge, 12 is the ground electrode, and 14 is the RF 711 source. Note that DC discharge may be used instead of RF discharge, but in that case, the thin film is formed by sputtering plasma onto the electrodes 10 and 12. Contamination is likely to occur.Furthermore, the ground electrode 12 may not be particularly provided, and the entire wall of the reaction chamber 2 may be used as a ground electrode to cause discharge.

16 is a substrate stand, 18 is a substrate such as alumina, 20 is an inlet for gas used for discharge, and 22 is a gas reservoir.

24 is a microwave power source; 26 is a microwave plasma generation container connected to the microwave power source 24 via a waveguide;
A fused silica plate 28 is used for vacuum sealing the container 26. 30 is an electromagnetic coil, and 32 is a DC power source for controlling the potential of the microwave plasma in the container. That is, the container 26 and the reaction chamber 2 are insulated, and the reaction chamber 2 is connected to the container 26.
When a positive potential is applied to the reaction chamber 2, the positive ions of the microwave plasma in the container are accelerated and driven into the reaction chamber 2.

These ions form a beam and are irradiated onto the substrate. At the same time, when this ion beam arrives between the discharge electrodes 10.degree. 12, an RF discharge is triggered.

? Discharge may be triggered by natural diffusion of TiTa205 ions. However, this is inefficient. 34
is a wire-mesh-like grid that keeps its potential negative with respect to the container 26 and the reaction chamber 2, that is, with respect to both the microwave plasma generation section and the RF plasma generation section, and repels the electrons in the microwave plasma. and return it to the container.

Although a negative potential may be applied to the grid 34 from the power supply,
In the embodiment, only the grid 34 was insulated. In this way, because of the electrons in the plasma, the grid 3
Since a negative space potential is formed around 4, this space potential was utilized. By this operation, positive ions in the microwave plasma are selectively flowed into the reaction chamber 2.

In the example, the magnetic field of the container 26 was 1000 Gauss, its diameter was 10 cm, and its length was 40C11.

Further, the frequency of the microwave power source 24 was set to 2.45 GHz. Furthermore, the experiment was conducted with the voltage of the power supply 32, that is, the potential of the container 26 relative to the reaction chamber 2, set to 70V. Next, reaction chamber 2
had a cylindrical shape coaxial with the container 26, had a diameter of 30 cm, and a length of 300I11. 13, 56 for the RF power supply 14
A frequency of MHz was used, and the energy was 50W. In addition, the discharge electrode 10.12 will eventually have a diameter of 8cff+
The distance between the disks was 6.5 cm. The position within the reaction chamber 2 is indicated by the distance from the grid 34 in the Z-axis direction in the figure.

Formation of thin film Tetramethyl S n I 2 Mo from the gas inlet 20
A mixture of 1% oxygen and 88Mo1% oxygen was introduced, and the total pressure in the reaction chamber 2 was reduced to 1%
It was discharged as r. Other gases such as Ar may be mixed into this mixture. Note that at this pressure, when the RF7Ii source 14 is used alone, the discharge is slight and the film formation is extremely slow.

Tetramethyl Sn is an example of a low boiling point compound of Sn.

Microwave on the other hand? Microwaves are supplied from the u source 24 to the container 26. Due to this microwave, microwave plasma was generated in the container 26. A magnetic field was applied to the container 26 from an electromagnet 30, and the plasma was caused to move in a circular motion by the Lorentz force, thereby suppressing contact with the container wall. Further, since some of the electrons in the plasma move in a circular motion 4' in synchronization with the microwave, they are accelerated by the electric field of the microwave. Since no magnetic field acts on the axial movement of the container 26, the generated plasma flows into the reaction chamber 2 via the grid 34. At this time grid 34
Since it is insulated, a negative space potential is generated due to the accumulation of electrons in the surrounding space. Because of this space potential, electrons are repelled by the grid 34 and returned to the plasma. As a result, an ion beam flows into the reaction chamber 2, and its density can be controlled by the power of the microwave power source 4 and the energy can be controlled by the voltage of the DC power source 32. Since the container 26 is maintained at a positive potential with respect to the reaction chamber 2, the positive ions in the plasma that have passed through the grid 34 are accelerated and enter the reaction chamber.

A flow of ions from the microwave plasma flows to the discharge electrode 1.
Reached 0.12 hours? 1, this ion triggers an rIF discharge. Therefore, the discharge starts even at a pressure at which RF discharge would not normally occur, or even if the RF main power supply power is low. Decompose the mixed gas of the Sn a-organic compound and oxygen by electric discharge to form a thin film of metal oxide on the substrate I8.

The purpose of creating microwave plasma in the container 26 is to
ion beam irradiation and triggering of RF discharge. Therefore, other excitation sources may be used, such as electron beams or ion beams.

2? [If a child beam is used, oxygen cannot be included in the thin film source gas because of the oxidation of the filament of the beam source.

Figure 2 shows the groove structure of the board. In the figure, 40 is a heat-resistant insulating substrate made of alumina or the like, 42 is a heater pattern made of Ru Oy or the like, and 44, 46, 48 are electrodes. These holes are masked with a photoresist 50. 52 is a hole provided in the photoresist. A thin film of metal oxide is formed in this hole 52. A large number of undivided substrates 40 were bonded together and used as the substrate 18 for forming a thin film.

After forming the thin film, the photoresist is removed, the thin film is heat treated, the substrate 18 is divided into individual sensors, and the electrodes 44, 46, 48 are wired to complete the gas sensor.

FIG. 3 shows the relationship between the formation time of a 5nap thin film and the sheet resistance of the thin film. The grounding position of the substrate 18 is the discharge electrode 10.12.
The pressure of the air-fuel mixture at the center of the cylinder was set to 3XIO-3Torr. (In principle, the same applies below.) The results shown in the figure are the obtained S
The n02 thin film was treated in air at 700°C for 1 hour, and after 1 hour,
The resistance value was measured at 280°C. Further, the surface temperature of the substrate 18 during the process of forming this thin film was about 8 cm, which was sufficiently lower than the allowable temperature limit (200° C. or lower) of the photoresist. It can be seen from the figure that the time required to form the thin film is 20 minutes or more. Therefore, below, the thin film formation time is 40
It was set as 1 minute.

Figure 4 shows the (discharge time/10
min, source gas pressure 3,0 X l O-3 Tor
r) shows an X-ray diffraction diagram. In the figure, (A) shows the thin film immediately after formation, and (B) shows the film after heat treatment at 700° C. for 1 hour in air. Note that for X-ray diffraction, quartz glass was used as the substrate 18. The untreated thin film is amorphous, and even when treated at 700°C, the average crystallite diameter (diameter) of 5no2 is 70~+00
/'It's only 1. That is, according to the present invention, a thin film that is amorphous or has a small average crystallite diameter can be easily obtained.

FIG. 5 shows an electron micrograph of the Sno thin film at 5000 times magnification. The film 1'7 has a thickness of about 0.2 .mu.m, and the state of the film has been heat-treated after manufacturing, so the substrate is quartz glass. 3. For manufacturing. Using a mixture of Ox l 0-3 Torr,
The film formation time is 40 minutes. In order to expose the substrate to iE, a crack was created in the film using a glass cutter (left side of the figure).

To determine the optimal pressure for thin film formation, the thin films were prepared by varying the pressure of the air-fuel mixture, and the sheet resistance at room temperature was measured. The results are shown in Table 1 in KΩ.

Table 1 (Optimal pressure) Z-axis position total pressure of substrate (Torr) 2cm l Ocm 1
8cm 26cm5XIO-'~10' Flash ■ ■3 x l O-' 20 6
0 160 -150I X I O-' o
ooooooooo.

*Discharge electrode 1 when the Z-axis position is +8c+n. Q, corresponds to the center of 12, and each data represents the sheet resistance of the thin film.

table! From the results, the pressure horn during discharge is 1x10-3~3
It can be seen that X I 0-2 Torr is preferable. Another feature that can be seen from Table 1 is that the resistance value changes relatively little depending on the substrate position (example of 3 X l O-3 Torr).
.

That is, in this invention, it is easy to form a uniform thin film.

The interaction between microwave plasma and RF discharge was investigated. If you operate only the RP main power supply and discharge without applying microwaves or magnetic fields, if you apply only microwaves and magnetic fields, RF7
In the case where the thin film was generated only by microwave plasma without using the No. 14 source, and in the case of the three examples
The sheet resistance of the two thin films at room temperature was determined. The results are shown in Table 2 in KΩ.

Table 2 (Interaction with microwave) 17F only 1083X1032X10'
5X103 microwave only 2X103 ■
lXl0′ ■Example 20
60 160 450* Reaction pressure is 3
X 10-3 Torr, the valley data represents the sheet resistance of the thin film.

From Table 2, it can be seen that thin films are formed by triggering RF discharge by microwave plasma.

With only 11F or microwave plasma, the thin film has extremely high resistance.

Special characteristics of each tanza SnO is amorphous or has a low degree of crystal growth.

Since a film was obtained, gas sensitivity and other characteristics were obtained that were different from those of conventional rays. The inventors investigated how the obtained gas sensor could be used to detect malodorous substances (mainly sulfur compounds and amine compounds such as L-T, S, and CH3S11).

Figures 6 and 7 show the results.

The obtained thin film (single S no 2) was heated at 360° in air.
Heat treated at 700°C for 1 hour and determined the gas sensitivity.

The results are shown as the ratio of resistance values in air and gas. The measurement temperature is 280°C. As a comparative example, S n(OC
A 5nOt film was used which was thermally decomposed by immersing the substrate in an isobutanol solution of 14*)* (OCaHsN 112). The thermal decomposition temperature was 500℃ and 700℃ due to thermal decomposition restrictions.
There were two types: ℃.

As is clear from FIGS. 6 and 7, the gas sensor of the example has higher gas sensitivity than the comparative example.

In particular, for S, the example was treated at 700°C, but for (CH”+)+N, the example was treated at 360°C.
Those processed with this method have high sensitivity. The heat treatment temperature in the example (treatment time 1 hour, in air) and the resistance value in air (2
80°C) is shown in Table 3.

Table 3 Resistance value in air (KΩ unit) Since the obtained SnO2 thin film becomes highly resistive when treated at high temperature, the upper limit of the treatment temperature is about 800°C.

Although the examples are shown for SnO2 thin films, if the raw material is, for example, a mixture of S b (CI) and oxygen, the proton conductor 5b205 thin IV5. Or get it.

Furthermore, if the raw material is Zr(CI-13)4, a ZrO2 thin film, which is an oxygen ion conductor, can be obtained. Further, in the embodiment, oxygen was added to the mixture and Sn was oxidized at the same time as the thin film was formed to obtain Snow, but it is also possible to form a Sn metal thin film and then oxidize it to 5nOt.

[Effects of invention This invention provides the following effects.

(1) When the discharge plasma is triggered by another excitation source to generate a discharge, the rise in the surface temperature of the substrate has been suppressed. This makes it possible to use photoresist and form fine thin films. This allows the gas sensor to be made smaller.

(2) It is possible to obtain a thin film that is amorphous or has a small average crystallite diameter, and has characteristics different from gas sensors using thin films with advanced crystal growth.

(3) Since there is little variation in thin film properties depending on the position of the substrate, a large number of gas sensors can be manufactured at once.

(4) When the excitation source is microwave plasma, oxygen can be added to the reaction gas. On the other hand, if an electron beam is used as the excitation source, the filament will oxidize, making it impossible to discharge in an oxygen-existing system.

(5) When a magnetic field is applied to the microwave plasma generation part,
It suppresses loss due to the plasma vessel wall, and the resonance between the electrons in the plasma and the microwaves allows the electrons to be accelerated by one speed.

(6) If a grid is provided between the microwave plasma generation part and the discharge part and the potential of the grid is negative, the electrons in the microwave plasma will be returned to the plasma, and the microwave will be efficiently absorbed into the plasma. You can do it.

(7) Since the microwave plasma and RF plasma used can be controlled independently, their density, energy, etc. can be adjusted freely, and thin film conditioning conditions can be easily controlled.

(8) If the raw material gas used for discharge is a mixture of a low-boiling metal compound and oxygen, a thin film of metal oxide can be obtained from the beginning.

(9) When obtaining a thin film of -SnO, if the low boiling point compound of Sn is SnC+4 or tetramethyl Sn, a vapor of the Sn compound having a high vapor pressure and a pressure suitable for forming a thin film can be obtained.

(10) When the thin film formation time is set to 20 minutes or more, the resistance value of the thin film does not change much depending on the discharge time, and thin films of substantially the same quality can be manufactured from different lots.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of the thin film manufacturing apparatus used in the example, Figure 2 is a plan view of the substrate used in the example, and Figure 3 is a characteristic diagram showing the resistance value of the metal oxide thin film obtained in the example. It is. Figure 4 (
Δ) and (B) are X-ray diffraction patterns of metal oxide thin films obtained in Examples, respectively. FIG. 5 is an electron micrograph showing the surface structure of the 5n02 thin film obtained in the example. FIG. 6 and FIG. 7 are characteristic diagrams of the gas sensor obtained in the example. In the figure, 2 reaction chambers, +0. +2? Ii pole, 141' (F power supply, 18
Substrate, 24 microwave power source, 26 microwave plasma generation container, 30 electromagnetic coil, 34 grid.

Claims (11)

[Claims] (1) In a method for manufacturing a gas sensor in which a metal oxide thin film is obtained by exposing raw material gas for the metal oxide thin film to discharge plasma, the plasma discharge is triggered by another excitation source. A method for manufacturing a gas sensor. (2) The method for manufacturing a gas sensor according to claim 1, wherein the excitation source is microwave plasma. (3) Applying a magnetic field to the microwave plasma generation part,
3. The method of manufacturing a gas sensor according to claim 2, wherein the microwave plasma is prevented from dissipating and the electrons are accelerated by resonance between the electrons in the microwave plasma and the microwave. (4) A grid is provided between these two plasmas, and the potential of the grid is set to be a negative potential with respect to these two plasmas, so that the electrons in the microwave plasma are repelled by the grid. 4. The method of manufacturing a gas sensor according to claim 3, wherein the gas sensor is returned to the microwave plasma. (5) The method for manufacturing a gas sensor according to claim 1, wherein the excitation source is an electron beam. (6) The method for manufacturing a gas sensor according to claim 1, wherein the excitation source is an ion beam. (7) Claim 1, characterized in that the raw material gas for the metal oxide thin film is a mixed gas of a low boiling point metal compound and oxygen.
The method for manufacturing the gas sensor described in . (8) Add the mixed gas to SnCl_4 and Sn(CH_
3) The method for manufacturing a gas sensor according to claim 7, wherein the gas is a mixed gas of at least one member of the group consisting of _4 and oxygen. (9) The method for manufacturing a gas sensor according to claim 1, characterized in that a mask is formed on a heat-resistant insulating substrate using a photoresist, and a metal oxide thin film is formed on the substrate after the mask is formed. . (10) The total pressure of the gas in the plasma discharge part is 1×10^-
2. The method of manufacturing a gas sensor according to claim 1, wherein the pressure is ^3 to 3 x 10^-^2 Torr. (11) The method for manufacturing a gas sensor according to claim 1, characterized in that the time for forming the thin film by plasma discharge is 20 minutes or more.