JP2546340B2 - Moisture sensitive element and its operating circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a humidity sensitive element and an operating circuit thereof. More specifically, the present invention relates to a humidity sensitive element based on a MOS type FET, which has improved moisture sensitive characteristics, and an operating circuit thereof.

[Conventional technology]

Controlling the relative humidity in the air is very important in the precision industry, food industry, textile industry, building management, etc., and as a moisture sensing element for detecting it, conventional metals, semiconductors, (porous)
It is known to use materials such as metal oxides, electrolyte salts and polymer membranes.

However, the humidity-sensitive elements using these various materials are not in a satisfactory state because they are difficult to maintain or have problems in reliability and responsiveness.

The present inventor has previously found that as a moisture-sensitive element having no such problems, an organic amine compound or a mixture thereof with a halogenated hydrocarbon or a halogenated silane is formed on the surface of a lower electrode formed on an insulating substrate. Has proposed a thin film moisture sensitive element in which a plasma polymerized film is formed and an upper electrode having moisture permeability is placed on the upper surface of the plasma polymerized film (Japanese Patent Laid-Open No. 1-10).
9251 and 1-121745). The plasma-polymerized film formed here has the effect that the capacitance of a capacitor using it as a dielectric, that is, the dielectric constant of the polymer film, changes in response to changes in relative humidity over a wide range.

Conventionally, as a method of detecting this type of capacitance change, a method of combining a CR oscillation circuit and a humidity sensitive element and converting a change in capacitance of the dry / wet element into a change in oscillation frequency has been used. . However, in the future, miniaturization of the sensor,
A method of combining with a semiconductor element such as a transistor as a signal conversion element is considered to be effective in achieving multi-functionality.

As such a method, a structure in which a capacitor having a moisture sensitive film as a dielectric is directly formed on the gate part of a MOS type FET is used, and the AC component of the gate voltage changes according to the permittivity of the moisture sensitive film. It has been proposed that a moisture sensitive member and a semiconductor signal conversion element can be integrated for this reason, and that the moisture sensitive element has been successfully downsized (electronic material 19
(August 1984, p. 127).

FIG. 4 of the drawing shows an equivalent circuit thereof,
In this circuit, the capacitance Cs of the capacitor with the humidity sensitive film as the dielectric and the AC component of the gate voltage The relation with and is shown as the following equation (1), and corresponds to Cs The principle is to detect the change in the current as a current change between the source and the drain.

However, such a method has the following problems. That is, from the above equation (1), Cs The relationship between and is as shown in the graph in Fig. 5, but as shown here, with respect to changes in Cs Changes linearly corresponding to the value of Cs being 10 ⁿ (F)
It stays within the range of one digit centered on. Therefore, when the capacitance of the humidity sensitive element changes over a wide area, such a measurement circuit cannot provide a sufficient response.

On the other hand, the thin film moisture-sensitive element using the above-described plasma-polymerized film as the wet and dry film, which the present inventor has previously proposed, has a capacitance change of 2
Since it is as large as a digit or more, such a measuring circuit cannot be applied.

[Problems to be solved by the invention]

The object of the present invention is a moisture-sensitive film such as a plasma-polymerized film,
Therefore, it is to resist the moisture sensitive element based on the MOS type FET, which has improved moisture sensitive characteristics.

Another object of the present invention is to provide an operating circuit for a humidity sensitive element which can be sufficiently applied to measurement of improved humidity sensitive characteristics.

[Means for solving the problem]

The humidity-sensitive element of the present invention to achieve the above object is a MOS type FET provided with a source diffusion layer and a drain diffusion layer,
The gate electrode is formed of a moisture permeable electrode, which is directly connected to the source electrode, and a moisture sensitive thin film layer is provided between the moisture permeable gate electrode and the insulating layer.

Furthermore, the operating circuit of such a humidity sensitive element is composed of a source diffusion layer,
The operation state of the field effect transistor constituted by the drain diffusion layer and the gate electrode is configured to be a depletion type in which current flows even when the voltage between the gate electrode and the source electrode is 0V.

FIG. 1 of the drawings is a vertical cross-sectional view of one embodiment of a moisture-sensitive element according to the present invention, showing a semiconductor substrate 1, insulating layers 2, 2 ′, 2 ″, a source electrode 3, a drain electrode 4 and a gate electrode 5. And MO provided with source diffusion layer 6 and drain diffusion layer 7
In the S-type FET, the gate electrode 5 is formed of a moisture permeable electrode,
A moisture sensitive thin film layer 8 is provided between the moisture permeable gate electrode 5 and the insulating layer 2 while directly connecting it to the source electrode 3.

The source diffusion layer and the drain diffusion layer are formed on the semiconductor substrate by diffusing impurities in the semiconductor substrate by thermal diffusion or ion implantation. At this time, the following impurities are selected depending on the conductivity type of the semiconductor substrate.

N-type substrate: boron, aluminum, gallium, indium P-type substrate: phosphorus, arsenic, antimony On the surface of the semiconductor substrate on which the source diffusion layer and drain diffusion layer are formed, source electrode-source diffusion layer and drain electrode-drain The insulating layer is formed except for the contact surfaces of the diffusion layer. The insulating layer is formed as a thin film of an inorganic nitride such as silicon nitride having excellent insulating properties and chemical stability or an inorganic oxide such as silicon oxide, aluminum oxide and tantalum oxide. The formation of these thin films can be performed by any method such as various conventionally used CVD methods and sputtering methods.

However, in any case, in order to make the interface between the gate electrode surface and the insulating layer electrically stable,
It is necessary to form a thin oxide film (film thickness of 100 to 1000Å) between the substrate and insulating layer. The formation of such an oxide film is usually performed by a thermal oxidation method. That is, the oxide film can be easily formed by heating the substrate to about 800 to 1000 ° C. in a dry oxygen atmosphere.

Generally, in order to make the operation state of the MOS type FET depletion type, it is necessary to form a channel on the surface of the gate portion in advance. However, when the conduction system of the substrate is P-type, an N-type inversion layer, that is, an N-channel is formed on the surface of the gate electrode due to the presence of metal ions in the oxide film and dangling bonds (dangling bonds), The operation state of is a depletion type. In the case of an N-type substrate, in order to form a P channel on the surface of the gate electrode, the same impurities as those used for forming the source / drain diffusion layer must be implanted in a very small amount. As described above, it is advantageous in terms of process to use the P-type substrate that can simultaneously form the N channel when forming the oxide film.

The insulating inorganic thin film formed to a thickness of about 500 to 10000Å as described above is once formed so as to cover the entire substrate surface, but in order to expose the electrode-diffusion layer contact portion, The insulating film in that portion is removed by a normal photolithography method.

That is, a photoresist is coated on the surface of a substrate, a positive image or a negative image is exposed so that only the contact portion is exposed, contact exposure is performed, and after development processing, the insulating film at the contact portion is removed by etching. Etching can be performed by either a wet method or a dry method. For example,
As the dry etching, a commonly used plasma etching method, reactive ion etching method, or the like is used. In the case of plasma etching, for example, about 5
Using CF ₄ containing ~ 10% oxygen as an etching gas, a high frequency (13.
The etching rate varies depending on the mating material, but when the mating material is SiN or SiO, it is generally about 50 to 400 Å / min.

Next, after forming the source electrode and the drain electrode in contact with the respective diffusion layers by a conventional method, a moisture sensitive layer is formed on the insulating layer between the source electrode and the drain electrode.

The moisture-sensitive layer is generally formed with a film thickness of about 1 μm or less in order to increase the response speed to humidity changes, from a material whose dielectric constant changes with the amount of adsorbed moisture according to relative humidity. As such a material, for example, porous aluminum oxide, cellulose acetate, polyimide, polyvinyl alcohol, or the above-mentioned plasma polymerized product proposed by the present inventors is used.

A gate electrode is formed on the upper surface of the sensitive layer.
The gate electrode is preferably formed of gold or platinum having excellent corrosion resistance, but it is required to have permeability so that water vapor such as in the air can reach the sensitive film. For this reason, the gate electrode is formed by the vacuum deposition method, and when the film thickness becomes thinner than about 250 Å, it shows a porous state, so it is desirable to set it within the range of about 50 to 250 Å. When forming such a porous electrode, a method of introducing a small amount of an inert gas such as argon or nitrogen into the vacuum vapor deposition atmosphere is also taken as an effective means.

When forming the gate electrode by this vapor deposition method, the source electrode 3
The gate electrode 5 and the source electrode 3 are formed by using an evaporation mask having an opening evaporation window on the side surface of the moisture sensitive layer 8.
And 9 can be short-circuited.

In the moisture sensitive element of the present invention configured as described above, the operating state of the field effect transistor including the source diffusion layer, the drain diffusion layer and the gate electrode is
It is used as a depletion type device in which a current flows between the source electrodes even when the voltage between them is 0V.

An equivalent circuit of this humidity sensitive element is shown in FIG. 2 of the drawing.

In this equivalent circuit, the gate electrode and the source electrode are directly connected, and the FE is connected between the source electrode and the drain electrode.
The voltage V _DS that satisfies Eq. (2) so that T operates in the saturation region
Is applied.

| V _DS | ＞ | V _GS −V _GS (th) ｜ (2) V _GS : Gate-source voltage V _GS (th): I _DS (source-drain current) in depletion type MOS FET In the saturation operation region of the MOS type FET where ≠ 0, which is the threshold value required to make ≈0, the transfer characteristic showing the relationship between V _GS and I _DS is as shown in the following equation (3). Can be approximated.

β: A constant determined by the element, which is determined by the geometrical structure of the gate, and is expressed by equation (4). μ: Electron or hole mobility ε: Dielectric constant W: Channel (gate) width To: Gate insulating film thickness L: Drain-source distance (gate length) In formula (4) above, except μ Is a value that determines the electrostatic capacitance Ci of the gate insulating film, and is represented by the following equation (5).

The following equation (6) is derived from the equations (4) and (5).

On the other hand, since V _GS is short-circuited between the gate and the source, V _GS = 0, and therefore, formula (7) is derived from formulas (3) and (6).

Further, since the moisture sensitive layer 8 is directly provided on the insulating layer 2 of the gate portion, the change in the capacitance Cs of the moisture sensitive film can be regarded as the change in the gate insulating film Ci. Therefore, the above equation (7) can be expressed as the following equation (8).

Here, μ, L, and V _GS (th) are constants peculiar to the FET, so in equation (8), I _DS and Cs have a proportional relationship, and Cs
A sufficient response of I _DS can be obtained for a wide range of changes in.

[Operation] and [Effects of the Invention] In the operating circuit of the humidity-sensitive element according to the present invention, the gate electrode and the source electrode of the humidity-sensitive element are electrically directly connected, and a constant amount is provided between the source electrode and the drain electrode. When a voltage is applied, the voltage between the source and drain becomes 0V because the gate and source are short-circuited, but since the operating state of the MOS type FET is the depletion type, the current according to the transfer characteristics peculiar to the FET is Will flow between the source and drain.

In this state, the capacitance of the moisture sensitive film can be regarded as a part of the capacitance of the insulating film of the gate portion, so if the capacitance of the dry and wet film changes according to the change in relative humidity, the FET The apparent capacitance of the gate insulating film changes according to the transfer characteristics of the gate insulating film, which causes the current between the source and the drain to change. Since the current value at this time changes in proportion to the capacitance of the gate portion, sufficient responsiveness can be obtained even when the capacitance changes over a wide range.

Further, in such an operating circuit, the gate electrode and the source electrode of the moisture sensitive element are short-circuited, so that the circuit configuration is simple, and in particular, a direct current or an alternating current power supply is not required between them, which simplifies the measuring device. Can be planned.

〔Example〕

 Next, the present invention will be described with reference to examples.

Example A MOS type FET having an n type source diffusion layer and a drain diffusion layer and having a depletion type operation state was formed on a P type silicon semiconductor substrate. The insulating layer is made of Si with a thickness of 5000 Å
It is formed from an O ₂ thin film.

N, N, N ', N'-tetramethylethylenediamine (0.0.7Torr) -methyl bromide (0.01T) is formed on the gate insulating film.
A plasma polymerized film having a film thickness of 5000 Å was formed as a moisture sensitive film by plasma polymerization under the discharge condition of power of 40 W and time of 30 minutes using an orr) mixed gas.

On this moisture-sensitive film, a vapor deposition mask with a vapor deposition window of the same shape and size as the gate electrode pattern was placed on a stainless steel plate (thickness 0.2 mm), and gold was vacuum-deposited over the mask to a film thickness of 200 Å. A gold vapor deposition film was formed and used as a gate electrode. Note that the gate electrode has a pattern shape which is connected to the source electrode on the substrate.

The moisture-sensitive element thus manufactured was used to measure the humidity-sensitive characteristics according to its operating circuit.
As the voltage between the drain electrodes, a voltage (12 V) sufficient to drive the FET in the saturation region was applied. The results obtained are shown in the graph of Fig. 3, which indicates that the relative humidity
It is clear that a satisfactory response is obtained over a wide area of 10-80%.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of one mode of the moisture sensitive element according to the present invention, and FIG. 2 is its operating circuit. FIG. 3 is a graph showing the relationship between relative humidity and output in the example.
FIG. 4 is an equivalent circuit of a heretofore known moisture sensitive element.
The figure is a graph showing the moisture-sensitive characteristics thereof. (Explanation of symbols) 1 ... Semiconductor substrate 2 ... Insulating layer 3 ... Source electrode 4 ... Drain electrode 5 ... Gate electrode 6 ... Source diffusion layer 7 ... Drain diffusion layer 8 ... Moisture sensitive layer 9 ... ... Gate electrode-source electrode short circuit

Claims (2)

(57) [Claims] 1. A MOS type FET provided with a source diffusion layer and a drain diffusion layer, wherein a gate electrode is formed of a moisture permeable electrode and the moisture permeable gate electrode and the insulating layer are directly connected to the source electrode. A moisture sensitive element having a moisture sensitive thin film layer interposed therebetween. 2. The operating circuit of the moisture sensitive element according to claim 1, wherein the field effect transistor constituted by the source diffusion layer, the drain diffusion layer and the gate electrode is operated such that the voltage between the gate electrode and the source electrode is 0V. An operation circuit that is a depletion type in which a current flows even when.