JPH01223339A - Gas sensor utilizing section of multilayer thin film - Google Patents

Abstract

PURPOSE:To enable highly sensitive detection of various gases, by a method wherein a low voltage is applied to two layers of metal electrodes close to each other to decompose a gas molecule such as water vapor adsorbed on the surface of the electrodes directly in a electrochemical fashion while a current flowing is monitored. CONSTITUTION:Two layers of metal electrodes 7 and 8 which can be manufactured by exposing a section part 6 of a multilayer thin film with a structure of laminating metal thin films 2 and 4 and insulator thin films 3 and 5 alternately are provided on a ceramic substrate 1 close to each other at a distance of several hundreds - several thousands A. A required voltage source is connected between the electrodes 7 and 8 to decompose a gas molecule adsorbed on the surfaces of the electrodes 7 and 8 directly in an electrochemical fashion under the introduction of a gas such as water vapor or ammonia. Thus, a current flowing between the two layers of the electrodes 7 and 8 is monitored thereby enabling highly sensitive detection the gas introduced.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a gas sensor necessary for the control of electrical equipment such as humidity and harmful gas detection, gas leak detection, and pollution prevention. By monitoring the current that flows through direct electrochemical decomposition of gas molecules adsorbed on the gas, it is possible to obtain a gas sensor that is highly sensitive, highly selective, highly versatile, and easy to design. .

(Conventional technology) Conventionally, gas sensors used to detect substances in the gas phase include semiconductor gas sensors, porous ceramic sensors for humidity measurement, solid electrolyte sensors, and electrochemical gas sensors. Various types of sensors suitable for various target gases have been developed, and some of them have been put into practical use.

However, in such conventional sensors, ■ Poor selectivity ■ Sensitivity is not sufficient ■ Sensitivity decreases after long-term use ■ Not easy to design ■ Maintenance is not easy Other issues remain.

(Problems to be Solved by the Invention) Porous ceramic sensors and electrochemical gas sensors are examples of conventional gas sensors that have been developed with the aim of solving the problems of conventional gas sensors and are related to the present invention. It will be done.

However, these porous ceramic sensors measure the change in electrical resistance due to physical adsorption of the target substance adsorbed to the pore structure of the oxide, so it is difficult to achieve the optimal pore distribution and various problems arise. However, there are many drawbacks such as the inability to provide selectivity to other gases.

In addition, electrochemical gas sensors monitor the current caused by the bridge reaction that occurs on the electrode surface, but because they require the use of an electrolyte, they not only have maintenance difficulties such as dissipation and absorption of water in the electrolyte, but also There were drawbacks such as restrictions on the amount of gas that could be produced.

(Means for Solving the Problems) An object of the present invention is to eliminate the above-mentioned conventional drawbacks and solve the problems by utilizing an indirect phenomenon such as a change in electrical resistance due to gas adsorbed on an element. Rather, it is constructed so that the adsorbed gas molecules can be directly electrochemically decomposed without an electrolyte, and can be measured using a cross-section of a multilayer thin film.The two-layer metal electrode is connected to an insulator thin film. A completely new type with high sensitivity and high selectivity that monitors the current that flows when a voltage is applied between 2N metal electrodes while gas is introduced, using electrodes that have a structure in which they are placed close to each other by several hundred to several thousand angstroms. gas sensors.

The present invention is a gas sensor that electrically detects various gases, which can be manufactured by exposing the cross section of a multilayer thin film having a structure in which metal thin films and insulating thin films are stacked alternately on a ceramic substrate. When a required DC voltage source is connected between two layers of metal electrodes consisting of an exposed cross section of a multilayer II thin film held close to a distance of 1,000 angstroms by an insulating thin film, and a gas such as water vapor or ammonia is introduced. Relating to a gas sensor using a cross-section of a multilayer thin film configured to detect introduced gas by directly electrochemically decomposing gas molecules adsorbed on the electrode surface and monitoring the current flowing between two layers of metal electrodes. .

The gas sensor using the cross section of the multilayer thin film of the present invention is characterized by performing an electrochemical reaction either in a high resistance solvent containing no electrolyte, in a gas atmosphere, or at low temperature.

Another object of the present invention is to form a multi-thin film structure in which metal thin films and insulating thin films are alternately stacked on a ceramic substrate with a thickness of several hundred angstroms to several thousand angstroms, and to A portion is removed by chemical etching or argon ion etching, and the two metal electrodes of the multilayer thin film are exposed in close proximity at a distance of several hundred to several thousand angstroms, and a required voltage source is connected between these 2N metal electrodes. When a gas such as water vapor or ammonia is introduced onto the electrode surface, the gas molecules adsorbed on the electrode surface are electrochemically decomposed, and the introduced gas can be detected by measuring the current flowing between the two metal electrodes. An object of the present invention is to provide a method for manufacturing a gas sensor using a cross section of a multilayer thin film, which is characterized in that the gas sensor is manufactured.

The gas sensor using the cross section of the multilayer thin film of the present invention has two layers of metal electrodes that are very close to each other at a distance of several hundred to several thousand angstroms and are completely electrically insulated from each other. It was made by using a cross section of a multilayer thin film made by stacking alternately insulating thin films.

Specifically, a part of such a multilayer thin film is made by exposing a part using chemical etching or argon ion etching and photolithography technology, and a low voltage of several volts is applied between the two layers of metal electrodes. By electrochemically decomposing various gas molecules adsorbed on the electrode surface and monitoring the current flowing between two layers of metal electrodes, gases such as water vapor, ammonia, and acetic acid can be detected at room temperature. The device is characterized in that it is configured to detect gases such as carbon dioxide that condense on the electrode surface.

(Example) The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an example of the configuration of the main parts of a gas sensor using a cross section of the multilayer thin film of the present invention. First, a 5iOz substrate 1 having a very flat surface with irregularities of several nanometers or less or a mirror-polished silicon wafer covered with a thick Si0g film is prepared. A cross section perpendicular to the surface of the multilayer thin film obtained by stacking multiple thin films ranging in thickness from several hundred angstroms to about 1 micron on such a smooth insulating plate is cut out.

FIG. 1 shows the structure of the electrode thus produced.

In the present invention, the multilayer thin film has a structure in which metals 2.4 and insulators 3.5 are alternately stacked on a substrate 1.
Insulators include silicon oxide (SiOt), aluminum oxide (A1203), and silicon carbide (SiOt).
C) Various materials such as silicon nitride (SiN) can be used, and metals such as -nickel (Ni), platinum (Pt), gold (Au), ffl (Ag), aluminum (AN), etc. Various substances can be used. 2
When obtaining a cross section with metal electrodes of layers, on top of the insulator substrate metal layer 2 - insulator N3 - metal)! ! 4-Thin films must be deposited in the order of insulator jW5. Further, in order to establish electrical connection with each of the two metal thin films, each metal thin film must be deposited in a planar pattern as shown in 7.8.

When using silicon oxide as the insulator layer and nickel as the metal layer, silicon oxide is deposited using plasma CVD on a substrate heated to 300"C from a mixed gas of silane (Sit14) and nitrogen oxide (NtO), and the metal layer is deposited using plasma CVD. The layers are deposited at room temperature using a high frequency sputtering device from various metal targets.The cross section 6 of the multilayer thin film is exposed by removing a portion (indicated by 9) of the multilayer thin film by chemical etching or argon ion etching. In the case of chemical etching, the surface of the multilayer film is partially covered with photoresist, and the exposed part is dissolved in a mixture of ammonium hydrogen fluoride (NH, F, IIF) aqueous solution and nitric acid to expose the cross section. In the case of argon ion etching, the surface of the multilayer thin film is partially covered with photoresist, silicon oxide is deposited thickly on the entire surface, the photoresist is peeled off, and finally the entire surface is etched with argon ions in a high frequency sputtering device. When sputtering is performed, the parts where the resist is removed are not covered thickly with silicon oxide, so the multilayer thin film is removed only in those parts and the cross section is exposed. Since it is necessary to apply a voltage, the two metal layers must be completely electrically insulated, and care must be taken not to break down the insulation when exposing the cross section.

The gas sensor that utilizes the cross section of the multilayer thin film produced as described above has the characteristic that the distance between the two metal electrodes can ultimately be made close to about 100 angstroms; The distance between the metal electrodes 4 can also be freely changed by changing the thickness of the silicon oxide thin film 3 sandwiched between the metals. Due to the feature that the distance between the two layers of metal electrodes is very small,
The gas sensor using the cross section of the multilayer thin film of the present invention can perform an electrochemical reaction even without an electrolyte, and furthermore, it has become possible to perform an electrochemical reaction simply by introducing a gas such as water vapor. For example, if the metal path HD is 130 nanometers, that is, the thickness of the silicon oxide thin film 3 between the two metal thin films 2.4 is 1300 angstroms, and the thickness of the two metal thin films is 1000 angstroms, When a voltage of about 3 volts is applied between two layers of metal electrodes, almost no current flows under vacuum, but when water vapor is introduced there, a voltage greater than 1.6 V is applied, that is, in an aqueous solution containing an electrolyte. A current flows due to the decomposition of the water adsorbed on the electrode surface at almost the same voltage as the voltage required for water electrolysis as seen in , and the amount of current increases rapidly as the voltage increases. The current density calculated from the area of the exposed nickel thin film electrode is very large, reaching 70+sAcm-'' when a voltage of 3.5V is applied.The flowing current gradually increases as the water vapor pressure increases, and the current density increases as the water vapor pressure increases. This shows that the gas sensor used works as a humidity sensor.As the distance HD between the two layers of electrodes increases, no response is observed, but this clearly shows the effect of the very small distance between the electrodes. Note that if this electrode is immersed in a sodium sulfate (NazSO4) aqueous solution in advance to adsorb ionic species on the electrode surface, the response to water vapor will increase, and the response will become visible from a lower water vapor pressure.

The gas sensor using the cross section of the multilayer thin film of the present invention responds not only to water vapor but also to gases such as acetic acid, formic acid, ammonia, etc., but also responds to gases such as methanol, ethanol, acetone, benzene,
It hardly responds to gases such as n-hebutane, 1-butanol, and acetaldehyde, even if their vapor pressures are very high. Furthermore, the dependence of the response current on the vapor pressure of the responding gases is different, indicating that the present invention works as a gas sensor with selectivity for various gases. Furthermore, by changing the types of insulators and metals used, it becomes possible to create gas sensors with various selectivities.

The gas sensor using the cross section of the multilayer thin film of the present invention also responds to carbon dioxide and the like at low temperatures.

The electrode of the present invention, in which ions have been adsorbed from an aqueous solution of sodium sulfate on the electrode surface, is heated to 50% carbon dioxide.
When cooling in the presence of 0 Torr, a current response is observed in a temperature range of -50°C to -30°C and an applied voltage of about 3V or more.

(Effects) As is clear from the above explanation, when a gas sensor using the cross section of the multilayer thin film of the present invention is used, by applying a low voltage (several volts) to two metal electrodes in close proximity to each other, By directly electrochemically decomposing gas molecules such as water vapor adsorbed on the electrode surface and monitoring the current flowing at that time, it becomes possible to detect various gases.

The gas sensor using the cross section of the multilayer thin film of the present invention can detect gases such as carbon dioxide that aggregate on the electrode surface at low temperatures by applying voltage to the two-layer metal electrode while cooling the entire electrode. Can be done.

Other effects of the gas sensor using the cross section of the multilayer thin film of the present invention are as follows.

(1) By changing the thickness of the insulating layer, that is, the distance between the two layers of metal electrodes, it is possible to create electrodes that are sensitive to various gas concentration regions, and by combining them, the total concentration of a certain gas can be adjusted. Gas sensors can be designed with high sensitivity over a wide range of regions.

(2) When using a certain type of combination of an insulating layer and a metal layer, some gases can be adsorbed to the electrode surface and others cannot, and even the adsorbable gases differ in their ease of adsorption, so the current The gas concentration dependence of the response is different, and it is possible to have selectivity in that it is sensitive to a specific gas. This means that by changing the types of materials used in the insulating and metal layers, it is possible to change the types of gases that can be adsorbed on the electrode surface, making it possible to selectively detect various gases at the same time. It has the advantage of being able to design gas sensors.

(3) By making many tiny holes in a multilayer thin film using microfabrication technology, it is possible to dramatically increase the cross-sectional area of the exposed thin film, which dramatically increases the amount of current that can be passed through the electrodes. It can be used not only as an electrode but also as an electrode for electrochemical reactions. For example, organic synthesis in highly resistant non-aqueous solvents, which has been difficult until now,
It can be used for electrochemical reactions of gases and electrochemical reactions at low temperatures where solvents cannot be used.

[Brief explanation of the drawing]

FIG. 1 is an enlarged view showing the structure of a main part of a gas sensor using a cross section of the multilayer thin film of the present invention, and FIG. 2 is a plan view of the same part. 1... SiO□ substrate 2.4... Metal thin film 3.5... Insulator thin film 6... Cross section of multilayer thin film 7... Lower layer metal thin film 8... Upper layer metal thin film 9
・・・Removed part of multilayer thin film

Claims (1)

[Claims] 1. A gas sensor that electrically detects various gases is manufactured by exposing the cross section of a multilayer thin film having a structure in which metal thin films and insulating thin films are alternately stacked on a ceramic substrate. In a gas sensor that electrically detects various gases, it is equipped with two layers of metal electrodes that are placed close to each other at a distance of several hundred to several thousand angstroms.
Connect the required voltage source between the metal electrodes in the layer, and directly electrochemically decompose the gas molecules adsorbed on the electrode surface while introducing a gas such as water vapor or ammonia, and monitor the current flowing between the metal electrodes in the two layers. A gas sensor using a cross section of a multilayer thin film, characterized in that it is configured so that introduced gas can be detected by doing so. 2. A gas sensor using a cross-section of a multilayer thin film, which is characterized by carrying out an electrochemical reaction in a high-resistance solvent that does not contain an electrolyte, in a gas atmosphere, or at low temperatures. 3. Form a multi-thin film structure in which metal thin films and insulating thin films are alternately stacked to a thickness of several hundred to several thousand angstroms on a ceramic substrate, and part of the multilayer thin film is chemically etched or etched with argon ions. The metal electrodes of the two layers of the multilayer thin film are exposed in close proximity at a distance of several hundred to several thousand angstroms, and a required voltage source is connected between these two layers of metal electrodes, and water vapor is applied to the surface of the electrodes. , we plan to manufacture a gas sensor that can detect the introduced gas by electrochemically decomposing gas molecules adsorbed on the electrode surface when a gas such as ammonia is introduced, and measuring the current flowing between both metal electrodes. A manufacturing method for gas sensors that utilizes the cross-section of a characteristic multilayer thin film.