JPS6312252B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a FET gas sensor. Sensors using gate-insulated transistors (FETs) are manufactured using IC technology, so they are easy to miniaturize and multiplex, and are expected to be used as sensors for medical and biochemical applications. There is. By changing _the sensitive membrane on the gate surface of a sensor using FET, it is possible to create a sensor that is sensitive to various chemical substances such as electrolytes, gases, enzymes, enzyme substrates, _etc. gas sensors are important as medical monitors. A gas sensor using FET as shown in Figure 1 has been proposed.
That is, in the figure, 1 is a drain electrode, 2 is a source electrode, 3 is a gate electrode, 5 is a gate insulating film, and 6 is a drain electrode.
is a channel, and 4 is a gas-sensitive film that also serves as a gate electrode, that is, a gate electrode having gas sensitivity. When the gas dissolves in the gas sensitive film 4 and reaches the interface with the gate insulating film 5 by diffusion, the interface potential changes, thereby changing the conductivity of the channel. Therefore, by measuring the conductivity of the channel, the gas concentration can be determined. However, since the gas-sensitive film also serves as a gate electrode, it must be made of an electrically conductive material. If the material is insulating, the electric field effect will not work uniformly across the channel, and the signal will become unstable. Metals are generally used as such gas-sensitive membranes, but since the gas diffusion rate in metals is extremely slow, the response speed of such sensors is generally extremely slow. Therefore, it is necessary to heat the entire body to increase the gas diffusion rate, and its uses are extremely limited. Currently, this type of FET gas sensor only has a hydrogen sensor. This is because hydrogen diffusion in palladium, which forms the hydrogen-sensitive membrane, is abnormally fast compared to other metal-gas systems, and no other gas has been found to have a practical response speed. In order to eliminate these drawbacks, it is possible to use porous electrodes, but this is not preferable because the strength of the electrodes decreases, and in the case of reactive gas sensors such as oxygen sensors, the electrodes deteriorate rapidly. . The present inventors have completed the present invention as a result of various studies in order to eliminate such drawbacks of the gas sensor and allow gas to quickly reach the FET sensing section. The present invention provides a gas diffusion layer with a thickness of 200 Å to 1 μ having a gas contact surface between the gate electrode and the gate insulating film, and the gate electrode is formed of a gas-sensitive conductor, or the gate insulating film is It is characterized in that the surface of the membrane is coated with a gas-sensitive membrane, and the gas is allowed to enter the gas diffusion layer from the gas-contact surface of the gas diffusion layer.
It is a FET gas sensor. The present invention is illustrated in FIG. 2.
That is, 7 is a gas diffusion layer, and gas enters the gas diffusion layer from the gas-contact surface of the gas diffusion layer 7 (the side surface and the top end in FIG. 2) and diffuses as shown by the arrow, and the gas sensitive film 4 (which also serves as the gate electrode 3 formed of a gas-sensitive conductor in FIG. 2) is changed. Since the gas diffusion layer can be made of an insulating material such as silicone rubber, the gas easily reaches the surface of the gas-sensitive film and exhibits a fast response. At this time, the gas naturally diffuses between the gas diffusion layer 7 and the gate insulating film 5, so it is necessary to select a combination in which the potential at the interface between 5 and 7 does not change depending on the gas concentration. Conversely, as shown in FIG. 3, the surface of the gate insulating film 5 is coated with a gas-sensitive film 4, and the gate electrode 3 is made of a conductor that is not sensitive to gas.
(selected according to the detected gas from the description on page 561)
The same effect can be obtained by The FET used in the present invention is commonly used.
Although FET sensors can be used, for example,
It is preferable to use a structure in which the electrode part and the gate part are separated, as described in No. 66194. It is important that the gas diffusion layer coated on such a FET gate part has a high gas diffusion rate and that it is a uniform thin film so that the gate electrode can exert a sufficiently stable electric field effect on the channel. Materials that meet the above conditions include rubbers such as polybutadiene, polyisoprene, polychloroprene, and silicone rubber, olefin polymers such as polyethylene, polypropylene, and polymethylpentene, fluorine-based polymers such as Teflon, polyhexachloropropylene, and porous ceramics. However, silicone rubber is extremely superior in that it has a high gas permeation rate. Such a gas diffusion layer is required to be very thin and uniform. The thickness is preferably 0.02-1μ, most preferably 0.1-0.3μ. If it becomes thicker than this, the electric field effect of the gate electrode will no longer reach the channel, resulting in extremely unstable output, and if it is too thin, diffusion in the film will be insufficient. Methods for manufacturing such membranes include the casting method,
There are plasma polymerization methods, sputtering methods, etc.
Ion beam sputtering is an excellent sputtering method because it allows the membrane to be made thin and uniform, and it is preferable that there are not many crosslinks (extremely dense crosslinks reduce gas permeability). Various materials can be applied to the gas sensitive membrane 4. For oxygen sensors, metals such as silver, gold, platinum, antimony, chromium, thallium, tantalum, etc.
It is formed from metal oxides such as Ga ₂ O ₃ , ZnO, Tb ₂ O ₃ , CoO, and Tm ₂ O ₃ , and organometallic compounds such as phthalocyanine and hemoglobin. In the case of other gas sensors, materials can be selected, for example, according to the descriptions in the Catalyst Handbook. When an insulator such as a metal oxide is used as the gas-sensitive film, it is preferable that the sum of its thickness and the thickness of the gas diffusion layer be within 1 μm. If it exceeds this, the output tends to become unstable, as mentioned above. Further, as described in connection with the gas diffusion layer, these film thicknesses must be uniform.
Such a film can be formed by ordinary sputtering, ion beam sputtering, high frequency sputtering, or the like. In the present invention, the shape of the gate electrode is preferably selected as follows. Since the gate electrode is a good conductor with a small diffusion coefficient, it prevents gas diffusion. Therefore, the width of the gate electrode influences the response speed, since the gas enters from the contact surface between the side surface of the gas diffusion layer and the gas at the end of the upper surface, as indicated by the arrow in FIG. For this reason, it is preferable that the gate electrode be located only above the gate portion as much as possible, and that it should not extend laterally to a wider width than that. The width of the gate electrode is preferably 1 mm or less, most preferably 50 to 200 microns.
Further, in order to form a gas contact surface with the gas diffusion layer 7, the gate electrode may have holes 8 and slits 9 as shown in FIGS. 4a and 4b, but the size of such holes and slits If the thickness exceeds twice the total thickness of the insulating film, a uniform electric field will not be applied to the channel, which is undesirable. Such a gate electrode can be formed by vapor deposition using a mask or by photo-etching after vapor deposition. A sensor in which a gas diffusion layer, a gas-sensitive film, and a gate electrode are provided in a gate-insulated FET in this way (if a gas-sensitive conductor is used for the gas-sensitive film, the gas-sensitive film can also serve as the gate electrode). can be measured by a circuit similar to other FET sensors. However, for measurements in living organisms, aqueous solutions, and wet conditions, it is necessary to cover this sensor section with a gas-permeable membrane. The same material used for the gas diffusion layer can be used for the gas permeable membrane, but
The thickness may be much thicker than that of the gas diffusion layer, and can be made up to several microns for Teflon and several hundred microns for silicone rubber, so it can be made by a normal dipping method. The FET gas sensor created in this way allows for stable gas concentration measurements with extremely fast response speed. The present invention will be specifically explained below using Examples. Example 1 A 5000 Å silicone rubber film was coated with Shin-Etsu Silicon KR205 toluene solution by the dip coating method on the gate portion of a FETPH sensor having a silicon nitride gate insulating film as described by Esashi et al. in the February 1978 issue of Ionics. Next, chromium metal was coated to a thickness of 600 Å using a high frequency sputtering method.
A gate electrode was connected to this using conductive paste. This sensor was tested in air and nitrogen with a source follower circuit similar to the method of Esashi et al _.
When we measured the gate-source potential (gate-source potential), we found that it was 66 mV lower in air than in nitrogen;
2 response time was approximately 30 seconds. On the other hand, when chromium metal was sputtered directly onto silicon nitride without coating it with silicone rubber, there was no sensitivity. Example 2 After coating the gate part of the FETPH sensor used in Example 1 with a 2000 Å Teflon film by high-frequency sputtering, a silver-epoxy conductive paste (Fujikura Kasei D-723S) was applied to the gate part and used as a gate electrode. . The thickness of the gate electrode was approximately 100μ. Using this electrode, we measured V _GS in various atmospheres and obtained the results shown in the table below.

[Table] This result shows that the sensor is not sensitive to CO ₂ and acts as an oxygen sensor. Also, the 1/2 response time of this sensor was 27 minutes. If an epoxy conductive paste electrode is attached directly to the gate without coating with such a Teflon film, the oxygen concentration will decrease.
At 20%, the response was only 10 mV, and the 1/2 response time was 2 hours, which was clearly inferior to the case without the gas diffusion layer.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a conventional FET gas sensor, and FIGS. 2 and 3 are cross-sectional views showing an example of the FET gas sensor of the present invention. Figure 4a
and FIG. 4b is a schematic diagram showing an example of the shape of the gate electrode in the FET sensor of the present invention. In the figures, the numbers indicate the following: 1...
Drain electrode, 2... Source electrode, 3... Gate electrode, 4... Gas sensitive film, 5... Gate insulating film, 6... Channel, 7... Gas diffusion layer, 8...
Hole, 9...slit.

Claims (1)

[Claims] 1. A gas diffusion layer of 200 Å to 1 μm having a gas contact surface is provided between the gate electrode and the gate insulating film of the gate insulated field effect transistor, and the gate electrode is formed of a gas-sensitive conductor. , or an FET gas sensor, characterized in that the surface of the gate insulating film is coated with a gas-sensitive film, and the gas is allowed to enter the gas diffusion layer from the gas-contact surface of the gas diffusion layer.