JPS63289443A - Moisture sensing element - Google Patents

Abstract

PURPOSE:To achieve a higher sensitivity sensitivity and response and a lower power consumption in detection, by arranging an element construction using a microbridge, cantilever or diaphragm structure while a sensitive film having several protrusions and recesses is provided. CONSTITUTION:A bridge-shaped thin film insulation layer 2 is formed on an Si substrate 1 and then, a microbridge 3 is formed having a hollow construction between a portion below an insulation layer bridge section and a substrate 1 by crystal axis anisotropic etching of Si as substrate 1 to make an element construction excellent in a lower heat capacity. Ideal for an insulation layer material is those lower in the heat capacity and close in the modulus of thermal expansion to the substrate 1, for example, SiO2, Si3N4, Al2O3 and ZrO2 and a cantilever or diaphragm structure is applicable in the shape. Moreover, a sensitive film 4 having protrusions and recesses on the surface thereof is formed on the microbridge 3 by performing a microprocessing using chemical and physical methods to increase the surface area of the sensitive film per unit area. The sensitive film material can employ Ge, SiC, TaN or the like as thin film material with a large thermistor constant.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an absolute humidity sensor that can directly detect the amount of water vapor in humidity measurement. The present invention relates to a moisture sensitive element.

<Conventional technology and its problems> Many types of humidity sensing elements or humidity sensors have been developed in the past, and in particular sensors for detecting relative humidity in the atmosphere are based on the electrical resistance value or static The following methods are mainly known, which take advantage of the fact that capacitance changes in response to moisture or water vapor in the atmosphere.

■Iron oxide (Fe2O3+Fe304), tin oxide (Sn
02) etc., ■Those using a hydrophilic polymer membrane or polymer electrolyte, or even fiber polymer, ■Solithium chloride L
Examples include those using electrolyte salts such as iC1), and (2) those in which conductive particles or fibers such as carbon are dispersed in a hygroscopic resin or a hygroscopic polymer membrane.

The above sensors have many advantages and disadvantages, such as detection humidity range, detection sensitivity and accuracy, response speed, reliability, and environmental resistance. When detecting a small amount of water vapor in an environment that changes rapidly, the following changes in relative humidity as a function of temperature can be considered, so there is a big problem in measuring humidity using the above-mentioned humidity sensing element or humidity sensor. In other words, assuming that the amount of water vapor in the detection atmosphere is constant and only the temperature of this atmosphere increases, the relative humidity will decrease due to the saturated water vapor pressure even if the amount of water vapor is constant; If the rise is rapid, it is expected that the slight increase in water vapor will be offset by the temperature change or the relative humidity will actually decrease, making it impossible to obtain results that reflect large changes in the amount of water vapor. Therefore, absolute humidity detection, which can directly detect the amount of water vapor, is considered to be more advantageous than relative humidity detection for measuring the humidity in the environment as described above.

As means for detecting absolute humidity (amount of water vapor), measurement devices that utilize the attenuation of microwaves by water vapor, the absorption of infrared rays, etc. have been conventionally used. Since these devices can directly detect water vapor using physical methods, they are advantageous for detecting small changes in water vapor even in environments that involve sudden temperature changes as mentioned above, but on the other hand, the configuration of the device including temperature compensation is large-scale. Therefore, the cost is also quite high. There is also a thermal conduction type absolute humidity sensor that uses two thermistors with the same characteristics, making use of the difference in thermal conductivity between humid air and dry air.It is small and has excellent environmental resistance, but compared to conventional However, it was not possible to obtain good output against minute changes in the amount of water vapor, and there were problems in terms of high detection sensitivity and high-speed response.

<Summary of the Invention> The present invention has been made in order to eliminate the drawbacks of the conventional moisture sensing elements as described above. The aim is to achieve higher detection sensitivity, faster response, and lower power consumption.

The method for detecting absolute humidity uses the aforementioned fact that the thermal conductivity of air depends on the amount of water vapor contained in the air.
The amount of water vapor is detected from the change in element temperature caused by the change in the amount of heat dissipated from the element as the thermal conductivity of the atmosphere changes.

That is, using two elements that are self-heated to a constant temperature, one (the first element) is exposed to the detection atmosphere, and the other (the second element) is exposed to the detection atmosphere.
By sealing the element in a constant humidity atmosphere, it is possible to detect a change in the amount of water vapor in the detection atmosphere with high precision as a differential output of a change in element temperature of the second element without being affected by the ambient temperature. Therefore, in order to achieve high sensitivity, high-speed response, and low power consumption, it is necessary to reduce the heat capacity of the element as much as possible and to provide an element structure that provides good heat dissipation.

Therefore, the humidity sensing element of the present invention not only miniaturizes the conventional thermal conduction type absolute humidity sensor, but also has a bridge fabricated by making full use of micromachining technology.

By forming a sensitive film on the cantilever or diaphragm, we aim to reduce the heat capacity, and furthermore, by creating multiple irregularities on the surface of the sensitive film or the surface of the substrate on which the sensitive film is formed using microfabrication technology, we can dramatically increase the heat dissipation area of the element. The device structure is designed to achieve this. In addition, as a sensitive film, Ge, SiC, T
Using a thin film of a material with a large thermistor constant such as aN,
Temperature changes in the element can be effectively output as resistance changes in the sensitive film. Furthermore, since detection is performed using a physical method, it is more stable against contamination of sensitive membranes and has better environmental resistance than detection using chemical methods that utilize changes in electrical properties due to adsorption and desorption of moisture. It also has excellent sex. In device manufacturing, all processes such as micromachining technology and sensitive film microfabrication technology can be batch-processed using normal semiconductor processes or their applied processes, resulting in excellent device reproducibility, compatibility, and low cost. It can be an element.

As described above, the present invention has a number of advantages as described above, and in particular, is excellent in high water vapor detection sensitivity and high-speed response, and is suitable for application as a food finishing sensor in a microwave oven. The purpose is to provide the following.

<Example 1. 〉 1st Doujin) β) is a schematic structural perspective view and a sectional view of a moisture-sensitive element showing one embodiment of the present invention. After forming the bridge-shaped thin film insulating layer 2 on the Si substrate l,
By performing crystal axis anisotropic etching, the substrate 1-insulating layer bridge portion 3 is etched under the insulating layer bridge.
(hereinafter referred to as a microbridge) The element structure has a hollow structure in between and is excellent in thermal insulation, that is, low heat capacity.Furthermore, a sensitive film 4 is microfabricated by chemical and physical methods on the microbridge 3. This increases the surface area of the sensitive film per unit area. Further, 5 is an electrode used for self-heating the sensitive film or evaluating the electrical resistance of the sensitive film.

To describe the manufacturing process of this device in detail, first, the rate of chemical etching differs depending on the orientation of the crystal axis.
i is a substrate, and a thin film insulating layer 2, which will become a micro bridge 3 and a mask during chemical etching, is formed on the substrate, the back surface, and the side surface of the substrate by thermal oxidation method, sputtering method, or depending on the material.
It is formed by a CVD method or the like, and microfabricated into a bridge shape by photolithography, chemical etching, or dry etching. In addition, considering the strength of the bridge, it is also useful to leave a predetermined thickness of Si under the insulating layer at the bridge part to form a bridge consisting of two layers, the insulating layer and the substrate material. By pre-diffusing or doping boron (B1, etc.) into the bridge part at a high concentration, that part becomes a stop layer during anisotropic etching (chemical etching), and as a result, a bridge made of the insulating layer and the substrate material is formed. In addition, the insulating layer materials are S i02 and S I 3N45AN20B.
, Z r02, etc., which have a low heat capacity and a coefficient of thermal expansion relatively similar to that of the substrate 1 are preferable, and a cantilever structure is also useful in terms of shape, and the cantilever may be formed of a two-layer material of an insulating layer and a substrate material. . Next, the sensitive film 4 is formed by vacuum evaporation.

The insulating layer is formed on the bridge portion by sputtering or CVD. Thereafter, the sensitive film 7 is microfabricated into a predetermined shape with one dimension, and the sensitive film 7 is shaped into a wave shape as shown in FIG. Perform fine processing. The shape does not necessarily have to be a trapezoidal protrusion like that shown in Figure 2 (B), but may also be a polygonal pyramid, a hemisphere, or a flat wave shape as shown in Figures 2 (B) and 3. Figure 3 β
) may have irregularities having a constant period. In addition to processing the sensitive film as shown in FIG. 4, if the insulating layer surface or the bridge is composed of two layers, the substrate material and the insulating layer, the surface of the base 9 on which the sensitive film of the substrate material is formed is processed to have an uneven shape. The sensitive film may be formed into a periodic uneven shape by performing microfabrication and depositing the sensitive film thereon along the unevenness. Furthermore, when the sensitive film is microfabricated into a predetermined shape and nine dimensions, a predetermined line width is obtained using photolithography technology and chemical or physical etching techniques, as shown in FIG.

It is also useful to perform fine processing in a zigzag (meandering) shape 12 consisting of lines, and FIG. 6 schematically shows a state in which a sensitive film 16 is formed in a meandering shape on an insulating layer cantilever 15. .

Alternatively, as described above, the substrate itself on which the sensitive film is to be provided may be micro-processed into an uneven shape, the sensitive film may be deposited along the unevenness, and then the sensitive film may be micro-processed into a meandering shape. In addition, as a sensitive film material, Ge is used as a thin film material with a large thermistor constant.

SiC, TaN, etc. can be used, and here Ge
is used. A thin film electrode 5 made of a metal material is formed by vacuum evaporation, sputtering, C2 method, etc. on the sensitive film on which the unevenness is formed by micromachining so as to obtain ohmic contact. The shape of the electrodes does not necessarily have to be a pair of opposing electrodes as shown in Figure 1. Depending on the specific resistance of the sensitive membrane, it may be a comb-shaped electrode, etc., and if it is in contact with the sensitive membrane, it can cover the upper and lower surfaces of the sensitive membrane. It may be formed in any of the following. Further, when the sensitive film is patterned in a meandering shape, the electrode only needs to be provided at the pad 20 portion schematically shown in FIG.

After the above steps, the substrate 1 (Sl) is heated for a predetermined period of time E, P.
, W (ethylenediamine-pyrocatechol/'-water)
When chemical etching, that is, crystal axis anisotropic etching, is performed using a solution such as Etching Solution, Si etching progresses in the preferential crystal axis direction from the part where there is no insulating layer that serves as a mask for the etching solution, and the insulating layer bridge pattern is As a result of removing the lower St, a microchip moisture sensing element having a microbridge structure is obtained in which the heat dissipation area is increased by forming fine irregularities on the surface of the sensitive film.

Next, the detailed operating mechanism as an absolute humidity sensor is as follows. As mentioned above, a thermal conduction type absolute humidity sensor that utilizes the fact that the thermal conductivity of the detection (measurement) atmosphere changes depending on the amount of water vapor contained in the atmosphere is comprised of two humidity sensing elements. is installed exposed to the detection atmosphere, and changes in the amount of water vapor in the detection atmosphere are propagated to the element as changes in thermal conductivity. On the other hand, the second element is kept in a constant humidity atmosphere, specifically, sealed with dry nitrogen or the like, and has a structure that prevents water vapor from entering from the outside. In this state, the first and second elements are heated to the same temperature and the amount of water vapor is detected. At this time, assuming that the amount of water vapor in the detection atmosphere is constant,
When the temperature of the atmosphere changes, the element temperature changes with the same width in both the first and second elements. However, regarding changes in the amount of water vapor in the atmosphere, the thermal conductivity of the surrounding atmosphere of the first and second elements differs depending on the water vapor. Since the influence of temperature is the same for both the first and second elements, as a result, the difference in the width of temperature change of the elements depends on the amount of water vapor. Therefore, by obtaining the differential output of the two elements, it is possible to detect the amount of water vapor with high accuracy regardless of fluctuations in ambient temperature. Further, an output signal from the element can be obtained by a change in the resistance of the sensitive film due to a change in the element temperature.

Based on the above operating mechanism, in order to obtain high sensitivity characteristics, high-speed response, and low power consumption, the heat capacity of the device must be maximized by forming a sensitive film on a substrate with excellent thermal insulation. It is important to reduce the size and increase the heat dissipation area of the sensitive film per unit area of the substrate, and the device of this example uses micromachining technology centered on anisotropic etching, photolithography technology, chemical etching, and dry etching. Micro-fabrication technology that makes full use of these techniques has enabled unprecedented water vapor detection sensitivity and high-speed response.

This makes it possible to achieve lower power consumption.

In this way, the humidity sensing element of the above example can directly detect the amount of water vapor for a thermal conduction type, and is suitable for application to an absolute humidity sensor, and has unprecedented high detection sensitivity and high-speed response, as well as low consumption. Since it has electric power and detects water vapor using a physical method, it has excellent environmental resistance and is useful for applications such as food finishing sensors in microwave ovens.

<Example 2. >> Go to FIG. 8) β) C1 is a schematic structural perspective view and a cross-sectional view of a moisture-sensitive element showing a second embodiment of the present invention. A thin film insulating layer is formed on the Si substrate and on the back and side surfaces of the substrate, and the insulating layer on the back side is etched in a predetermined shape with one dimension as a mask for anisotropic etching according to the pattern required to form the diaphragm. By performing anisotropic etching of Sl on which no layer is formed, a diaphragm 23 consisting of two layers, an insulating layer 22 provided on a Si substrate 21 and a Si substrate material, is formed. Further, the diaphragm has a sensitive film 24 patterned in a predetermined shape and one dimension, and a plurality of concave and convex portions in a predetermined shape and one dimension are formed on the surface of the sensitive film by microfabrication technology. Sensitive membrane 2
This structure provides a resistance of 4. The details of the device manufacturing method are almost the same as in Example 1 above, but the stop layer of the substrate material of the diaphragm portion is obtained by doping B or the like at a high concentration in advance. The diaphragm is therefore formed by two layers: this B-doped substrate material and an insulating layer. Furthermore, the thickness of the substrate material of the diaphragm part does not necessarily have to be controlled by doping with B or the like, but can also be controlled only by the anisotropic etching time. It is also possible to form a diaphragm using only layers. In addition, the uneven shape or the method for forming unevenness on the surface of the sensitive film is also described in Example 1.
A meandering shape similar to ゜ is also useful.

The diaphragm-type moisture-sensing element obtained in this way has a dramatic improvement in thermal insulation of the sensitive film, that is, a reduction in the heat capacity of the element, and heat dissipation from the element, compared to conventional heat-conduction type moisture-sensing elements. However, it is slightly inferior to bridge or cantilever type elements. However, considering the mechanical strength of the diaphragm, it is advantageous compared to bridges or cantilevers, and therefore has high sensitivity and fast response.

It has low power consumption and is useful for applications where mechanical strength is required.

<Effects of the Invention> The moisture-sensitive element according to the present invention has the following practically extremely useful advantages.

fi+ It can be applied to a thermal conduction type absolute humidity sensor and can directly detect the amount of water vapor, which is more advantageous than relative humidity detection especially when measuring humidity when the temperature of the detection atmosphere is accompanied by rapid changes.

(2) The element configuration uses a microbridge, cantilever, or diaphragm structure, which provides excellent thermal insulation of the sensitive film, which means that the heat capacity of the element is minimized, and the heat dissipation of the element is improved by providing multiple irregularities on the surface of the sensitive film. By dramatically increasing the surface area, water vapor detection has become more sensitive than ever before.

Achieves high-speed response and low power consumption.

(3) Since water vapor is detected by a physical method, it is stable against contamination of the element surface and has good environmental resistance.

(4) The device can be manufactured by batch processing using a normal semiconductor process or an application process thereof, has excellent reproducibility and compatibility, and can be an inexpensive device.

As detailed above, the humidity sensing element of the present invention is effective in detecting absolute humidity, can be manufactured at low cost, has good environmental resistance, and has an element structure that significantly reduces heat capacity and increases heat dissipation area. 1. It has many excellent characteristics such as high sensitivity detection characteristics, high speed response, and low power consumption operation, making it suitable for many applications such as food finishing sensors in microwave ovens. It is extremely useful in practice as a moisture-sensitive element.

[Brief explanation of drawings]

Fig. 1 (At(B+ is a schematic structural perspective view and cross-sectional view of a bridge type moisture sensing element showing one embodiment of the present invention. Fig. 2 W) 03+, Fig. 3 (3) β) and Fig. 4 This figure is a schematic perspective view showing an enlarged main part of the sensitive membrane of the moisture sensitive element shown in FIG. 1. FIG. 5 is a schematic perspective view showing a main part of a sensitive film patterned in a meandering shape as an example of the microfabricated structure of a sensitive film according to the present invention. FIG. 6K) CBl[C] is a schematic perspective view and a sectional view of a cantilever type moisture sensing element for explaining one embodiment of the present invention. FIG. 7 is a plan view showing one embodiment of the electrode pad when the sensitive film is microfabricated into a meandering shape on the bridge. FIG. 8 1) βIcI is a schematic perspective view and a sectional view of a diaphragm type moisture sensing element showing one embodiment of the present invention. 1.13.21...Substrate, 2.14.22...Insulating layer, 6,61,9.11...Base, 3,18...Bridge, 4.7,71.8,81 , 10.24... Sensitive film, 5, 25... Electrode, 12, 16. 19... Sensitive film (meandering shape), 15... Cantilever, 17.20... Electrode pad, 23 ...Diaphragm. Agent Patent Attorney Takeshi Sugiyama (and 1 other person) 0
Kyouichi';'Q! "1- (A') (B) Figure 8

Claims (4)

[Claims] 1. An element mounting portion having a bridge shape, a cantilever shape, or a diaphragm structure is formed on the insulating layer laminated on the substrate, and a sensitive film whose electrical properties change depending on a change in the amount of vapor is disposed on the element mounting portion, A moisture sensing element characterized in that the surface of the sensing film is formed into an uneven shape. 2. The moisture-sensitive element according to claim 1, wherein the sensitive film is patterned in a meandering shape. 3. 3. The moisture sensing element according to claim 1, wherein the underlying surface on which the sensitive film is layered is processed to have an uneven surface, and the surface of the sensitive film laminated on the uneven surface exhibits an uneven surface. 4. Claims 1 and 2 in which the sensitive film is constituted by a combination of a first film exposed to the detection atmosphere and a second film sealed in a constant humidity atmosphere vessel and to which the detection atmosphere temperature is transmitted. Moisture-sensitive element according to item 1 or 3.