JPS60168043A - Field effect type sensor - Google Patents

Abstract

PURPOSE:To suppress an influence exerted on an FET element by impurities and an ion, and to obtain a stable output characteristic for a long period of time by coupling as one body an induction body and the FET element, and providing an auxiliary electrode on an interface of the induction body and the FET element. CONSTITUTION:An n channel FET of a MOS type consists of a p type silicon substrate 1, a source 2, a drain 3, an SiO2 film 5, an electrode use electric conductor film 6, and an Si3N4 film 7, and between an induction body consisting of a humidity sensible body 9 and a gate electrode 10 having a humidity transmitting property on the gate insulating films 5, 7, and the FET element, a conductive blocking film 8 is provided in order to obtain a humidity sensor which is stable for a long period of time. An electrostatic capacity of the humidity sensible body 9 is varied in accordance with humidity in an open atmosphere, therefore, humidity is detected as a variation of a drain current under a constant voltage impressed to the electrode film 10. Since the film 8 is provided, it is obstructed theat impurities and an ion are diffused to the gate insulating films 5, 7, and a stable output characteristic is obtained for a long period of time.

Description

Detailed Description of the Invention [Technical Field] The present invention relates to a field effect transistor (hereinafter simply abbreviated as FET) such as a MOS type or MIS type. The present invention relates to a so-called FET5 sensor, which forms a sensitive body whose external characteristics change, and uses the sensitive body to detect changes in external factors as changes in the gate action of an FET.

<Background of the Invention> The degree of capacitance, electrical conductivity, or electrostatic potential is increased by chemical or physical interaction with the physical quantity to be detected.
i. The so-called FET type sensor, which combines a sensitive body that causes a gaseous change and an FET element, and grasps the physical quantity to be detected as a change in the gate action of the FET element, is
By skillfully utilizing the high input impedance of the T-element and its amplification effect, a high-output and small-sized sensor can be obtained, making it a practically preferable sensor. In particular, FET type sensors, which have a structure in which a sensitive body is formed on the gate part of an FET element, allow the element size to be set small, and it is possible to form an extremely large number of elements on the same substrate. This is a preferable form both for dogs and in terms of cost. However, in this case, the operational stability of the FET element, and hence the FET
It is necessary to pay attention to ensuring the stability of the output and the reproducibility of the characteristics as a type sensor. In other words, since the material of the sensitive body and the manufacturing method vary greatly depending on the type of sensor intended, special consideration must be given to the shape of a normal single FET element, and the operating characteristics of the FET element also vary. It varies greatly depending on the susceptor material and its manufacturing method. In particular, depending on the sensitive material, there is a possibility that it may contain a large amount of impurities or ions, or that impurities or ions may be mixed into the interface between the sensitive material and the gate insulating film during the process of forming the KPi reactive material on the FET element. Compared to the case of forming a single FET element, the FET
The operating characteristics of the element and the output characteristics of the FET type sensor tend to become unstable. Furthermore, when a so-called atmosphere sensor such as a gas sensor or a humidity sensor is configured as an FET type sensor, the sensor element is directly exposed to the outside environment, so the FE
This causes fluctuations and deterioration of T characteristics. In this way, the influence of impurities and ions in the sensor material, or impurities and ions mixed during the manufacturing process or during use, on the operating characteristics and sensor output of the FET element is suppressed, and the FET exhibits stable output characteristics over a long period of time. The use of FET type sensors is a common problem in converting various sensors such as gas sensors, humidity sensors, ion sensors, biosensors, or infrared sensors into FETs. In particular, FET-type gas sensors, humidity sensors, ion sensors, and biosensors require direct interaction with the sensing body, so it is not possible to cover the sensor element with a pancage, etc., so it is difficult to solve the above problem. It is extremely important. As one solution to the above problems, device structures have been developed in which a silicon nitride film with a small diffusion coefficient for ions and moisture is used as a gate insulating film, and the surface of the FET element is covered with a silicon nitride film. There are problems with long-term stability, and it is not always sufficient.

<Objective of the Invention> The present invention has been made based on the above background, and is aimed at preventing impurities and ions contained in the sensing body or the interface between the sensing body and the FET element, or mixed in from outside the element during use. FET that suppresses the influence of impurities and ions on FET element operation and output, and provides stable output characteristics over a long period of time.
The purpose of the present invention is to provide a type sensor.

<Embodiment> FIG. 1 is a structural sectional view of an FET type humidity sensor showing one embodiment of the present invention. FIG. 2 is an equivalent circuit diagram for explaining the operating principle of the sensor.

The FET element in this embodiment is a MOS type n-channel zlz FET, and an n-type source 2 and drain 3 are formed in parallel by diffusing phosphorus near the surface of a p-type silicon substrate 1. There is. A silicon substrate 1 is covered with a silicon dioxide film 5 having a through hole with a source 2 and a drain 3 .

The gate insulator 11 consists of a silicon dioxide film (51
02) 5 and a silicon nitride film (SiaN4) 7, the silicon nitride film 7 is further deposited on the silicon substrate 1 and the silicon dioxide film 5 with one end in contact with the source 2 and drain 3. It also covers the upper surface of the conductive film 6 for t[tk, and also functions as a protective film for the FET element. A moisture sensitive body 9 and a moisture permeable gate electrode film 1° are laminated on the gate insulating film 5.7, or a conductive film is formed at the interface between the moisture sensitive body 9 and the silicon oxide film 7. It has a structure in which a blocking film 8 is inserted. The blocking film 8 serves as an auxiliary ITJM for applying a voltage for canceling drift, which will be described later, to the moisture sensitive element 9. In this embodiment, the moisture sensitive body 9 is formed of a polyvinyl alcohol/l/film crystallized by thermal calcination or a cellulose film on acetate, but an organic or inorganic solid electrolyte film or a metal oxide film such as aluminum oxide is used. J#" may be used. The moisture-permeable gate electrode film 1o is a gold vapor deposited film with a thickness of about 100λ, and the blocking film 8 is a gold vapor-deposited film with a thickness of about 2. oooA gold or aluminum vapor deposited film was used. However, these element constituent materials are not necessarily limited to those mentioned above, and it is naturally possible to substitute other suitable materials. In addition to the moisture-sensitive body 9, gas-sensitive bodies, ion-sensitive bodies, and other substances sensitive to chemical substances, heat, light, etc. can be used.
As the element, an MIs type other than the MO8 type can also be used.

Next, the FE having the above configuration according to the equivalent circuit diagram in FIG.
The operating principle and features of the T-type humidity sensor will be explained. In the equivalent circuit diagram, capacitances Cs and Ci represent the capacitances of the moisture sensitive element 9 and the two-layer gate insulating film 5.7 in FIG. Further, RL indicates a load resistance coupled in series with the drain electrode 6, and RB indicates a resistance coupled in series with the blocking film 8.

First, in order to facilitate the explanation of the basic operation of the FET type humidity sensor, we will explain the case in which there is no blocking film 8 and the moisture sensitive element 9 is formed in direct contact with the gate insulating films 5 and 7, that is, in the equivalent circuit diagram. The case where there is no resistor RB will be described.

When the voltage applied to the moisture permeable gate electrode film 10 is VA and the threshold voltage of the FET element is vth, the drain current ID is given by the following equation.

However, in equation (1), μm represents carrier mobility, and L and W represent the channel length and channel width of the FET, respectively. Further, C is the capacitance when the capacitance Ci of the gate insulating film and the capacitance Cs of the moisture sensitive element 9 are coupled in series, and is expressed as follows. Therefore, by changing the capacitance Cs of the humidity sensing element 9 in accordance with the humidity in the surrounding air, humidity can be detected as a change in the drain current ID under certain conditions.

The above is the basic operating principle of the FET type humidity sensor.

However, in the above-mentioned operation, as a matter of course, there is a direct current potential difference between both sides of the moisture sensitive element 90, so if some impurity ions are present in the moisture sensitive element 9, the electric field may Movement, rearrangement and localization of these impurity ions occur. As a result, this has a significant effect on the channel section of the FET element in terms of element characteristics, causing fluctuations in the threshold voltage Vt11, and is a major cause of changes over time (drift) in the operating characteristics of the FET element and the output signal as a humidity sensor. becomes. A similar phenomenon occurs when impurity ions are present at the interface between the moisture sensitive body 9 and the moisture permeable gate electrode film 10 and at the interface with the gate insulating film 5.7. Moreover, as mentioned above, it is difficult to avoid the contamination of impurity ions from the external atmosphere, and therefore, solving the above problem is an extremely important issue in FET type temperature sensors.

A FET that basically solves the above problems and is stable for a long time.
In order to obtain a type humidity sensor, the structural feature of the FET type humidity sensor of this embodiment is as shown in FIG. This is due to the intervention of 8.

Then, as shown in the equivalent circuit diagram of FIG. 2, the blocking film 8 and the moisture permeable gate electrode film 10 coated on the surface of the moisture sensitive element 9 are coupled via a resistor RB, and the applied voltage VA is changed to a DC voltage. The FET element is driven by VA (DC) and an alternating current voltage VA (AC) of frequency f superimposed thereon. If the applied DC voltage VA(t)C) is sufficiently smaller than the withstand voltage of the gate insulating film and there is no leakage current from the gate insulating film, the direct current component of the pressure G will be applied to the effective gate t applied to the fronting film 8. VG(DC) is VA(D
C), and no direct current potential difference occurs on both sides of the humidity sensitive element 9.

Therefore, the above-mentioned phenomena such as movement, rearrangement, and localization of impurity ions are suppressed, and furthermore, the presence of the fronting film 8 prevents these impurity ions from diffusing into the gate insulating film. However, in this case, VG(DC) is always vA
(DC), it goes without saying that the humidity sensor cannot be operated only by VA (DC). The DC applied voltage VA (DC) functions to provide the optimum bias voltage in the ID-VG characteristics of the FET element. As a humidity sensor, in order to drive it, that is, the electrostatic field [iC
In order to detect changes in s due to humidity, an alternating current applied voltage VA (AC) is required.

Impedance of the humidity sensitive body at frequency f; (27If
Cs) 'Resistor RB having a sufficiently large resistance value compared to '
is coupled between the blocking film 8 and the moisture permeable gate electrode film 10, RB can be ignored and the alternating current component VG(AC) of VG is given by the following equation.

That is, under the condition of applying VA (AC) with a constant amplitude, VG (AC) changes depending on the value of the electrostatic field Cs of the humidity sensor, so the output signal as a humidity sensor is It can be extracted as the AC amplitude of ID.

FIG. 3 shows the output versus relative humidity characteristics of the FET type humidity sensor of the above embodiment. In addition, in FIG. 3, a heat-sintered acetyl cellulose film is used as the moisture sensitive element 9, and fixed resistors RB and RL are used.
are IOMΩ and IKΩ, respectively, and VA (DC) = 5V, V
This is an actual measurement example of output versus relative humidity characteristics at room temperature when driven at A(AC)=100 mVrms (10 KHz).

Next, as an experimental example showing the output stability of the FET type humidity sensor according to this embodiment, the relationship between the leaving time of the element left indoors and the sensor output at a relative humidity of 60% was actually measured and shown in FIG. For comparison, FIG. 4 shows a case (A) in which a blocking film 8 is used and a case in which a moisture sensitive body (acetylcellulose film) 9 is formed directly on the gate insulating film 7 without using a blocking film 8. The relationship between the standing time and the output is shown for each of (B). (a) Old: The driving conditions and measurement conditions of the pixel elements were the same, and each sensor output was based on the initial value. As seen in FIG. 4, it was demonstrated that the effect of the blocking film 8 was extremely large and that the sensor output was kept stable for a long period of time. In addition, the characteristics of FET elements, such as drain current (ID) vs. drain voltage (VD5) characteristics and drain current (ID) vs. gate voltage (VG) characteristics, do not change over time and have extremely excellent reproducibility. confirmed. On the other hand, when the blocking film 8 is not used, that is, in the case of (B) in FIG. 4, the ID vo of the F'ET element is
Both the s characteristic and the ID-■G characteristic undergo large changes over time,
The reproducibility of characteristics was also extremely poor. Moreover, v6
The ID VD of FET can also be changed by applying operations such as turning 0N-OFF or reversing the polarity of VG.
Since the S characteristics or ID-V6 characteristics are observed to exhibit phenomena that are significantly different from the initial characteristics, it is possible that the impurity ions present in the moisture sensitive body or at the interface between the moisture sensitive body and the gate insulating film are moved or regenerated by the electric field. It is interpreted that the effect of distribution (rearrangement) has a significant influence on the characteristics of the FET element.

<Effects of the Invention> As explained in detail in Examples 11 above, the present invention eliminates impurities and ions existing in the sensitive body or at the interface between the sensitive body and the FET element, and impurities and ions mixed in from the surrounding air during use. Because it has the effect of significantly suppressing the influence that it has on the output characteristics of FET elements and FET-type sensors, it is suitable for various types of biosensors, not only for humidity detection, but also for gas and ion detection, as well as organic substance detection. This brings great sound effects to the long-term stability and characteristic reproducibility of the operating characteristics and output characteristics of the FET type sensor.

[Brief explanation of the drawing]

FIG. 1 and FIG. 2 show embodiments of the present invention, respectively.
T-type jli! Structure W of the degree sensor (top view and equivalent circuit diagram). FIG. 3 is a characteristic diagram of the output versus relative humidity of the humidity sensor shown in FIG. 1, and FIG. 4 is a characteristic diagram showing the change in output over time. Curve A in the figure shows actual measured values for the FET type humidity sensor in FIG. 1, and curve B shows actual measured values for the FET type humidity sensor when no blocking film is used for comparison. ■ ・C) Core TrF plate 2 - Source 3 Drain 5 Silicon dioxide film 7 Silicon nitride film 8 Pronking film 9 Moisture sensitive element 1
0...Gate electrode membrane agent Patent attorney Aihiko Fukushi (and 2 others) b W; 1 Figure 0 20 40 60 80 100 a.

Claims (1)

[Claims] 1. A field-effect sensor in which a field-effect element is externally coupled to a sensitive body that causes an electrical change through physical or chemical interaction with a detected object. . A field effect sensor, characterized in that an auxiliary electrode for applying a voltage for canceling the drift of the sensitive body acts as an auxiliary electrode to the sensitive body. 2. The field effect sensor according to claim 1, wherein an auxiliary electrode is interposed at the interface between the gate insulating film of the field effect element and the sensitive body, and the gate electrode is disposed on the other surface of the sensitive body. 3. The field effect sensor according to claim 1 or 2, wherein the sensing body is a moisture sensitive body whose capacitance or electrical conductivity changes due to adsorption and desorption of water vapor or moisture. 4. The field-effect sensor according to claim 3, which uses a cellulose-based membrane, a vinyl-based (e.g. organic or inorganic) solid electrolyte membrane, or a metal oxide film as the moisture sensitive body.