JPS6358249A - Humidity detecting element - Google Patents

Abstract

PURPOSE:To improve the chemical resistance and gas resistance of a moisture sensitive element and to obtain a moisture detecting element which is produced with the fewer processes of production and has high reliability by changing the electrostatic capacity formed by sandwiching the polyimide film of a moisture sensitive material by a semiconductor substrate and thin metallic film with temp. CONSTITUTION:The rear surface of a silicon wafer 2 is subjected to a heat treatment to form an ohmic contact electrode 1 and thereafter, the polyimide moisture sensitive film 3 is cured. Gold is then deposited by evaporation over the entire surface through a metal mask which shields the peripheral part of the detecting element according to the shape of the electrode in order to provide moisture permeability thereto through the metallic film. Polyimide is coated as a protective film 5 over the entire surface and is subjected to a heat treatment. A hole 6 for taking out the upper electrode is formed on the protective film 5. Then the electrostatic capacity between the substrate 1 and the electrode 4 changes linearly with a change in relative humidity.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a humidity sensing element that detects the relative humidity of an atmosphere based on a change in capacitance value.

(Prior Art and Problems) Moisture sensing elements can be roughly classified into two types based on the materials used and their operating principles. One of them is an electrical resistance variable type that takes advantage of the fact that the electrical conductivity of porous ceramics made of sintered materials such as metal oxides changes with respect to humidity. In these materials, the environment in which the moisture sensing element is used is not limited to humidity.

Since it often contains oil and the like, the resistance value increases with the use period, and performance is likely to deteriorate due to so-called aging. In order to restore the performance of degraded elements, there is a method of performing thermal cleaning, but this method requires a heater and a control circuit.

The other type is a capacitance variable type, which utilizes the fact that the dielectric constant changes and the capacitance changes depending on the amount of moisture in the moisture sensitive material. Examples of moisture-sensitive materials include organic polymer films, metal oxide films, aluminum oxide films, and the like. Among these materials that have been announced so far, in terms of moisture sensitivity, the change in capacitance with respect to relative humidity does not necessarily change linearly, but deviates from a straight line in low or high humidity regions. There are also many. Further, even under the same humidity condition, many of them have so-called temperature dependence, in which the capacitance value changes as the ambient temperature changes. For this reason, when performing signal processing on the output of the humidity sensing element, it is inevitable that the circuit configuration becomes complicated. Generally, ponds using polymer membranes have the disadvantage that they cannot withstand use at high temperatures.

The present invention was made in order to solve the problems of the prior art, and its purpose is to improve the chemical resistance and gas resistance of a humidity sensing element, and to improve the electrostatic voltage V against changes in relative humidity. It is an object of the present invention to provide a reliable humidity sensing element that changes linearly, has no temperature dependence, and requires fewer manufacturing steps.

(Means for Solving the Problems) In order to solve the above-mentioned problems, selection of a moisture-sensitive film is important. Polyimide resins are generally heat resistant.

Excellent solvent resistance and cold resistance, as well as the electrical properties of the pond.
In particular, it is a well-known fact that it has excellent features such as a dielectric constant that is almost constant even with changes in temperature and a high volume resistivity. In addition, polyimide l (some types of fat exhibit a water absorption rate of several percent depending on the type of fat). These comprehensive advantages make it appropriate to use polyimide moisture-sensitive membranes as moisture-sensitive membranes.

Documents published on capacitive moisture sensing elements using polyimide resin include, for example, published patent publications.

There are No. 55-66749 and No. 60-166854. In the case of JP-A-55-66749, an adhesive layer is provided on a metal substrate, a polyimide layer is formed thereon, and a thin film of gold that is permeable to moisture is further formed. In this case, the upper gold electrode is likely to be damaged by contact with foreign matter, leading to changes in the characteristics of the element.

Next, in the case of JP-A-60-166854, a comb-shaped tantalum metal layer is provided on a glass substrate, the surface of this metal is oxidized to form an electrically insulating layer, a polyimide layer is formed on it, and then a polyimide layer is formed. This requires a complicated process in which the upper comb-shaped metal electrode is vapor-deposited on the part of the electrode that has been removed by etching, and then heat-treated. Furthermore, because the upper metal is exposed due to its structure, it causes deterioration of the moisture sensing element. To overcome these drawbacks, the inventors adopted a silicon semiconductor wafer as the substrate. The reason for this is that silicon itself can be used both as a rigid substrate and as a lower electrode.

Therefore, there is no need to particularly provide a lower electrode layer, which simplifies the manufacturing process. Furthermore, if the humidity sensing element is formed on a silicon substrate, it is possible to integrate a circuit element with the output signal processing function of the humidity sensing element and a temperature sensing element on the same substrate, which is expected to further expand the range of applications. It is.

Furthermore, since the humidity sensing element of the present invention is provided with a thin polyimide protective film layer on the top that is permeable to moisture, it can be protected from mechanical contact, dust, etc., and becomes a more reliable humidity sensing element.

(Example) This invention will be explained by showing an example below, but this invention is not limited thereto.
The figure shows a structural diagram of a humidity sensing element according to an example.

First, on the back side of a P-type or N-type silicon wafer 2 with a resistivity of 10 Ω·cIIl or less that has been surface-polished, aluminum is vacuum-deposited in the case of P-type, and gold is deposited in the case of N-type, and then heat-treated in vacuum. Then, the ohmic contact electrode 1 is formed.

Next, after thoroughly degreasing and cleaning the silicon wafer 2, a polyimide layer (adjust the fat to an appropriate viscosity, apply it using a spinner, and cure the polyimide moisture-sensitive film 3 at the temperature specified by the polyimide resin manufacturer. If this film is thick, the sensitivity of the moisture-sensitive characteristics will deteriorate.If it is thin, the resistance value of the film itself will be low, and the resistance value configured in parallel with the capacitance will be reduced, and signal processing This causes the above inconvenience.Generally 1.
A thickness of 0 to 5°0 μm is suitable.

Next, the upper electrode 4 is formed. In the process of forming this upper electrode, there are two methods depending on the shape of the electrode. One method is to evaporate gold over the entire surface through a metal mask that shields the periphery of the sensing element.
Limited to around A. Ike's method uses a metal mask that has a comb-shaped part on which metal is not deposited. In this case, since moisture permeates through the space, the metal may be of a different type, and the thickness may be considerably less than (100 OA or more).

In the next step, the entire surface is coated with polyimide as a protective film 5 to a thickness of 1 μm or less, and heat treatment is performed. A hole 6 for taking out the upper electrode is formed in the protective film 5. This method may be a general photo etching method using a photo resist.

Next, use a scriber to divide it into individual parts and complete.

(Effects of the Invention) Figure 3 shows the humidity sensitivity characteristics measured by measuring changes in capacitance with respect to changes in relative humidity. As shown in this figure, the moisture sensitivity characteristics change completely linearly. Also, the temperature characteristics are 20°C930°C
Even when the ambient temperature is changed to 140°C, there is almost no change in the moisture sensitivity characteristics and there is no temperature dependence. Hysteresis phenomenon was also not observed. Furthermore, despite the presence of a protective film, the humidity response is within 30 seconds for both moisture absorption and dehumidification, which poses no practical problem.

As described above, the humidity sensing element of the present invention is a humidity sensing element that has almost solved the problems of conventional products.

[Brief explanation of drawings]

FIG. 1 is a top view of a humidity sensing element according to the present invention, FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1, and FIG. 3 is a graph showing humidity sensitivity characteristics according to an embodiment of the present invention. 1... Ohmic contact electrode 2... Silicon wafer 3... Polyimide (polyimide) film 4... Upper electrode 5... Protective film 6
...Large tube for taking out the upper electrode Figure 1 Figure 3 Z6 40 60. 80
100+ mouth to n change (%)

Claims (1)

[Claims] A polyimide film as a moisture sensitive material, a metal thin film for an electrode, and a protective polyimide film are sequentially deposited on a semiconductor substrate, and the polyimide film as a moisture sensitive material is sandwiched between the semiconductor substrate and the metal film to form capacitance. A capacitive humidity sensing element that detects that the capacitance between the two changes depending on the humidity.