JPS63121739A - Gas sensor for nox - Google Patents

Abstract

PURPOSE:To obtain a sensor for gaseous NOx which can be used at a room temp. as well by installing a Langmuir-Blodgett film for adsorbing the gaseous NOx between a gate insulating film and gate electrode of an insulated gate type FET. CONSTITUTION:The MOSFET is prepd. and the gate part is held formed with only the insulating film 1 consisting of an SiO2 film. A thin film of Si3N4 or org. high polymer is usable in place of SiO2. The Langmuir-Blodgett film is built-up on the film 1 and is rested in the flow of iodine vapor diluted by the gaseous nitrogen. This resting is executed dividedly several times for about several tens seconds until the film exhibits a prescribed resistance value to form the sensitive film 3. The gate electrode 2 is formed on the film 3. The gaseous NOx diffuses into the gate electrode 2 and is adsorbed in the sensitive film 3 when such gas sensor is rested in a gas contg. the NOx. The interfacial potential corresponding to the amt. of the gas adsorbed is generated between the insulating film 1 and the sensitive film 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gas sensor for NOx. More specifically, the present invention relates to a NOx gas sensor using a Langmuir-Blodgett film capable of adsorbing NOx gas as a gas-sensitive substance.

[Conventional technology]

Conventionally, phthalocyanine-based metal complexes have been mainly used as gas-sensitive substances for Noz. However, when this metal complex is used as a sensitive substance, the measurement temperature must be set within the range of 100 to 300°C in order to promote adsorption and desorption of NOx gas, and therefore the temperature is high. Not only does it require special care in handling, but it also has disadvantages such as high power consumption.

[Problem that the invention seeks to solve]

Therefore, the present inventor conducted various studies in search of a NOx gas sensor that can perform measurements even at room temperature. For this purpose, a film formed by diffusing iodine from N-docosylpyridinium and tetracyanoquinodimethane by the Langmuir-Blodgett method exhibits high conductivity and is capable of adsorbing and desorbing NOx gas at room temperature. (Proceedings of the 2nd In
International Meeting on Ch.
The above-mentioned problem was solved by using a 5-sensors 7-06.1986) and installing it between the gate insulating film and the gate electrode of an insulated gate type FET.

[Means for solving problems]

Therefore, the present invention relates to a NOx gas sensor, and this gas sensor has a NOx gas adsorbing property in which a NOx gas adsorbing Langmuir-Prodget film is installed between the gate insulating film and the gate electrode of an insulated gate type FET. As the Langmuir-Prodgett film, N-docosylpyridinium 0(CH2)zt
A film obtained by diffusing iodine into a film formed by the Langmuir-Prodgett method from C.[11] and tetracyanoquinodimethane x:X.[■] is used.

The Langmuir-Prodgett film can be formed as follows. The above compounds [■] and [■]
The mixed solution is developed in a Langmuir-Prodjet tank filled with pure water (conductivity: 18 mΩ·CII+), and compressed to a surface pressure of 35 Nm/m. The Langmuir-Prodgett film is accumulated on the substrate by repeatedly lowering the substrate vertically into the film above the water surface using this surface pressure and then lifting it up again. The reciprocating speed of the substrate at this time was 60 cm/min when descending and 0.5 cm/min when rising, and by slowing down the speed during rising, drying of each accumulated film was ensured. A series of such accumulation steps are performed under room temperature conditions (21° C.) in a nitrogen atmosphere to form a cumulative film of about 20-30 layers on the substrate.

The diffusion of iodine into the Langmuir-Prodgett film thus formed is approximately 40 to 80 pp as described below.
It is carried out by placing it in a stream of iodine vapor diluted to a concentration of m.

Next, while referring to the drawings, we will introduce such a Langmuir-Prodgett film with NOx gas adsorption properties into an insulated gate type FE.
A NOx gas sensor configured by being installed on the gate insulating film of a MOS FET (T) will be described.

In a MOS FET, when a gate voltage is applied through the oxide film, the current flows between the source and drain depending on the gate voltage.
= Since the current flowing through the sensor changes, this characteristic can be used in gas sensors. That is, a gas-sensitive film is installed between the gate electrode and the oxide film, and when NOx gas is adsorbed to the sensitive film, the film itself becomes positively charged, which is measured from the current change between the source and drain. The purpose is to detect gas concentration.

FIG. 1 of the drawings is a perspective view showing the basic aspect of the NoI gas sensor according to the present invention, and is a typical MOS
In the FET structure, a Langmuir-Prodgett film 3 adsorbing NOx gas is provided between a gate insulating film 1 and a gate electrode 2 made of gate metal.

To manufacture this, after applying the necessary masking in each step,
This is done as follows.

First, a MOS FET is manufactured using a conventional process, with only the insulating film 1 made of Sun film being formed at the gate portion. The formation of the 5in2 film is performed by exposing the silicon substrate to a dry oxygen or water vapor atmosphere heated to about 900 to 1100°C to oxidize the substrate surface,
For example, the oxidation treatment is carried out in dry oxygen heated to 1050° C. for about 1 hour.

Even if it is not a 3in2 film, it is possible to use a thin film of Si3N4 or an organic polymer as long as it is a homogeneous insulating film with a thickness of about 200 to 2000, but it is difficult to adhere to the silicon substrate and have electrical affinity. In terms of properties, etc., the oxide film (Si2 film) obtained by direct thermal oxidation of a silicon substrate forms the most stable insulating film, and it is extremely difficult to form because it is simply exposed to an oxidizing atmosphere at high temperatures. It's easy.

Next, a Langmuir-Prodgett film is accumulated on this gate insulating film 1 as described above, and this is left in a flow of iodine vapor diluted with nitrogen gas. This neglect is
This is done several times for several tens of seconds, and the film is
A film having a resistivity of 10"Ω·Cm is formed thereon, and a sensitive film 3 is formed thereon.

Thereafter, a gate electrode 2 is formed on this sensitive film, and the gate electrode is formed by depositing a thin film of gold, platinum, etc. to a thickness of about 100 to 500 mm using sputtering, vacuum evaporation, etc. It will be done.

In this case, the electrode thickness is preferably formed as thin as possible so that the measured NOx gas can diffuse within the metal thin film and easily reach the sensitive film.

The gate electrode formed here may be a slit-shaped electrode 4, 4' as shown in FIG. 2 or a zigzag-shaped electrode 5 as shown in FIG. In these cases,
Since NOx gas directly reaches the sensitive film through the gaps in the electrode pattern, it becomes possible to increase the thickness of the electrode film and to use inexpensive aluminum as the electrode material.

[Effect]

In actual measurement, a method is used in which a bias is applied between the source 6 and the drain 7 to provide a constant current, and the interface voltage of the gate portion 8 is directly read from the bias voltage.

When this gas sensor is left in a gas containing NOx, such as nitrogen gas, the NOx gas diffuses through the gate electrode and is adsorbed on the sensitive film, and the amount of adsorption between the gate insulating film and the sensitive film varies depending on the amount of adsorption at this time. An interfacial potential is generated between the two.

〔Effect of the invention〕

It was confirmed that there is a proportional relationship between the NOx gas concentration and this interfacial potential output even under room temperature conditions.

That is, a gas sensor of the embodiment shown in FIG. 1 was produced under the following conditions: Oxide film formation conditions: 1050°C in dry oxygen for 1 hour Langmuir-Prodgett film = 30 layers Cumulative iodine gas concentration:
60 ppm Gate electrode: Gold, thickness Measured by 100 people Measured at a temperature of 21° C. The results shown in the graph of FIG. 4 were obtained.

From this, the gas sensor of the present invention using an iodine-diffusing Langmuir-Progett film as a sensitive film shows that the adsorption amount of NOx gas is concentration dependent even at room temperature.
It can be seen that a NOx gas sensor that can be used even at room temperature is constructed.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a basic aspect of the Noz gas sensor according to the present invention. 2 and 3 are perspective views of other embodiments. Moreover, FIG. 4 is a graph showing the NOx gas concentration-interfacial potential output characteristic. (Explanation of symbols) 1... Gate insulating film 2.4.5... Gate electrode 3... Sensitive film 6... Source 7... Drain 8 ·····Gate

Claims (1)

[Claims] 1. A NOx gas sensor comprising a Langmuir-Blodgett film adsorbing NOx gas between the gate insulating film and gate electrode of an insulated gate FET. 2. The NOx gas sensor according to claim 1, wherein the insulated gate FET is a MOS FET. 3. The Langmuir-Blodgett film capable of adsorbing NOx gas is formed by diffusing iodine into a film formed from N-docosylpyridinium and tetracyanoquinodimethane by the Langmuir-Blodgett method. The NOx gas sensor according to item 1.