JPS6371648A - Diode type humidity sensor - Google Patents

Abstract

PURPOSE:To obtain a diode type humidity sensor which needs not be put in a periodic refreshing operation by constituting the thin film of ZnO and a Schottky diode while using one of Cr, Ni, W, and Au which has a high work function and is not affected by reducing gas such as hydrogen as an electrode. CONSTITUTION:A 1st ZnO layer 1a with <=10<-2>OMEGAcm electric resistivity and a 2nd ZnO layer 1b with >=10<5>OMEGAcm electric resistivity are laminated in order on a lower electrode 2 formed by laminating Cr and Au in order and then kind of metal among Cr, Ni, W, Au, Pd, and Pt is laminated. The ZnO film 1 is formed by having its section in an inverted L shape, a part of it is joined with a glass substrate 4, and the lower electrode 2 is arranged in the gap between the ZnO film 1 and glass substrate 4. Further, the lower electrode 2 is formed of two layers, a Cr layer (100Angstrom thickness) is formed as 2a on the side contacting the glass substrate 4, and an Au layer (1,000Angstrom thickness) is formed as 2b on the Cr layer. An Ni layer (200Angstrom thickness) is formed as the upper electrode 3 on the ZnO film. Consequently, rectification characteristics corresponding to humidity are obtained by applying an about 2V AC voltage. Therefore, high-temperature refreshing operation is not required and sensor is miniaturized and power consumption is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a compact, low-power, maintenance-free humidity sensor.

(Prior art and problems to be solved by the invention) Conventionally, many humidity sensors have been developed that are made of physically and chemically stable ceramics.The ceramic moisture-sensitive structure has a structure in which ions are transmitted by physically adsorbed water. 4 Types that use gold [internal use and other 9 surface science, s, 189 cx9s2):
Shimizu et al., Electrochemistry.

so, 831 (x9sz): Sadaoka et al., electrochemistry,
51.437 (1983)) and a type that utilizes the change in electronic conductivity of semiconductors due to chemically adsorbed water (H, Arai
et al.

Proceedings of the Intern
ational Meeting on Chemica
l 5ensora+ 393 (1983): T,
N1tta et al.

Proceedings of the Intern
ational Meeting on Chemica
15sensors+387 (1983)). In the former type, humidity can be determined by physically adsorbing water into the pores of the ceramic, causing hydronium ions to become electrically conductive carriers and lowering the impedance of the element.

For this reason, the moisture sensitivity has the drawback that it depends greatly on the pore distribution and impedance of the element, and the water that has entered the pores becomes difficult to come out, causing errors when used for a long time. It had the disadvantage that it had to be heated and refreshed. The latter type usually has an operating temperature of 300 to 500°C and has the disadvantage of having to be used at high temperatures.Recently, sintered bodies of CuO and ZnO have been mechanically bonded to reduce humidity A method for measuring CuO/ZnO has been proposed (Nakamura et al. 1 Journal of the Chemical Society of Japan 1154 (1985))
This method takes advantage of the fact that the rectifying properties of the heterojunction change when water enters the CuO/ZnO interface. Although it has the advantage of not requiring high temperatures, it has the disadvantage that it is impossible to miniaturize it and integrate it with other elements on a chip because it is made into elements by applying a mechanical pressure layer to the sintered body. Was.

(Objective of the Invention) The present invention was proposed to improve the above-mentioned drawbacks, and includes refreshing at high temperatures required by conventional humidity sensors, miniaturization, which was impossible with conventional humidity sensors, and the need for other elements. It is an object of the present invention to provide a maintenance-free thin film diode-type humidity sensor that solves the problem of integration with the above object. The lower part is formed by sequentially horizontally forming Cr and Au! Extremely high electrical resistivity of 1O
After sequentially rolling the first ZnO layer with an electrical resistivity of -ffiΩ or less and the second ZnO layer with an electrical resistivity of 10'Ω or more, a Cr layer (Ni), a We (Au) layer of one type of gold (Pd+Pt) The gist of the invention is a diode type humidity sensor characterized by laminating S.

However, the most important feature of the present invention is that Cr and Ni have a high work function and are not affected by reducing gases such as hydrogen.
, W, or Au as the upper S electrode to form a Schottky diode with a thin film of ZnO. The conventional technology is different from the conventional technology in that the humidity sensor is constructed using a thin M[I, Schottky junction, and the upper electrode is made of Cr.
Due to the fact that it uses one of Ni, W, and Au and its operating mechanism, it does not require the regular refresh operation that is seen in ceramic humidity sensors that utilize ionic conductivity due to the absorption and absorption of water. The difference is that it is free.

Next, the present invention will be explained in detail. It should be noted that the embodiments are merely illustrative, and it goes without saying that various changes and improvements may be made without departing from the scope of the present invention.

FIG. 1 shows a first embodiment of the present invention, (
A) is a perspective view, and (B) is a cross-sectional view. In Figure 0, 0 shows a ZnO film, and this film has a two-layer structure with 9% 1a [air resistance 10'''30 layers]. The thickness is 1ooo
A, 1b The layer on the electrical resistance 106Ω side is laminated in one layer with a thickness of 1 μm. Thus, the ZnO film has an inverted cross-section, a part of which is bonded to the glass substrate 4, and the lower electrode 2 is disposed in the gap between the ZnO film and the glass substrate 4. Further, this lower electrode 2 is also formed in two layers, and the side in contact with the glass substrate 4 is made of Cr as 2a.
layer (thickness 100^) r On top of the Cr layer is A as 2b.
The u layer (1000λ thick) o3 is the upper electrode, and the Ni layer (200^ thick) is formed on the ZnO film.

To operate this, it is necessary to
An alternating current voltage of about 2 V is applied at a low frequency of about Hz. Lower electrode 2bi'i electrical resistance lO-! Zn0 layer 1a of Ωd
The lower electrode 3 is connected to the ZnO layer lb on the electrical resistance 106Ω side and forms a Schottky junction, so it exhibits rectifying properties. The upper S″QI 3 is a Ni layer with a thickness of 200A, and there are many gaps, making it a situation that is self-evident.
c. Since the surface of Z210J So1 is susceptible to water absorption, water is present at the interface between the upper electrode 3 and ZnO*1 in proportion to the humidity.

Because it has become such a legacy, 1. When an AC voltage of f12V is applied, rectification characteristics as shown in FIG. 2 can be obtained depending on the humidity. This rectification is due to the popping conduction of H generated by the scale center of chemically adsorbed water at low humidity, and is due to the low diffusion of H3O+ generated from physical water absorption at high humidity. In any case, due to the effect caused by the vomit decomposition reaction of water69, the water that has entered between the formations is decomposed and consumed. Therefore, there is no need for a refresh operation at high temperatures. As is clear from this result, compared to the conventional technology, it is possible to construct a humidity sensor using four membranes.
Moreover, it is small because it does not require refresh operation due to high temperature.

Improvements have been made that enable lower power consumption.

Note that the electrical resistivity of the first ZnO layer is preferably 101 Ωd or less, but if it exceeds 101 Ωcmf, it is difficult to form an ohmic connection with the lower electrode, which is not preferable. Further, the electrical resistivity of the second ZnOF is preferably 1080 or more, but if it is less than 10'Ω, it does not exhibit rectifying properties with respect to the upper electrode, which is not preferable.

[Example 2] In the element structure of the m1 diagram in Example 1, the lower electrode 3
Table 1 shows the moisture sensitivity and gas sensitivity to hydrogen gas when Cr+Ni, W, and Aug Pd are used.

Table 1 Bias +1.5v source I/I. Io Current value at 0% humidity In humidified current value Table 1, in dry air, forward bias 1.5
Calculate the electric a value when set to v. , the current value when humidity is applied and when hydrogen is added is defined as I/I
. Moisture sensitivity and gas sensitivity were evaluated. For reference, the results of the Ni upper electrode 3 in Example 1 are also shown.

In the case of only 70% humidity, moisture sensitivity was observed independent of the upper electrode, but when 1000 ppm of hydrogen was added to air with a humidity of 70%, only Pd reacted and high sensitivity was observed.

Although platinum-based metal thin films such as Pd and Pt react with reducing gases such as hydrogen at low temperatures, they can be used as temperature sensors in an atmosphere free of reducing gases. As is clear from this result, there was an improvement in that Cr HNl *W and Au sensed only humidity. [Example 3] FIG. 3 is a diagram explaining the third example of the present invention,
A platinum thermometer 6. is placed on the Si substrate 5. IC section 7. An electrode pad 8 is provided. Ni is used for the lower electrode 3 of the element portion. These operations include sensing moisture with the element part, sensing temperature with the platinum thermometer 6, calculating these in the IC part 7 to calculate the relative humidity, and passing the result of step 7 through the electrode pad 8. Output. In this way, an IC that can perform calculations
Part 7 and the platinum thermometer 6 and humidity sensor element part are the same-S.
The reason why the diode type humidity sensor of the present invention can be formed on the T substrate is that all of the diode type humidity sensors of the present invention can be formed with thin films and do not require refreshing at high temperatures. As is clear from this result, compared to the conventional technology, there was an improvement in miniaturization including the arithmetic unit and lower power consumption (Effect of the invention) As explained above, according to the present invention, ZnO Thin film and Cr
* Nl + W * A Schottky junction is formed with a thin film of Au, and the water that has entered the gaps between these junctions is completely electrolyzed, changing the rectification of the Schottky junction and measuring humidity. Unlike previous humidity sensors using ceramics, this sensor does not require an operation to expel water by refreshing at high temperatures, and because the element can be constructed using a thin film, it has the advantage of being maintenance-free, smaller, and with lower driving power. . Furthermore, since the St process can be used, M rusting can also be achieved.

[Brief explanation of the drawing]

Fig. 1 shows an example of the humidity sensor of the present invention, Fig. 2 shows the humidity sensing characteristics of the first element, and Fig. 3 shows the element shown in Fig. 1 and a platinum thermometer, and their information is processed and output. This shows a humidity sensor that integrates xcg. l...ZnO film 2...Lower 1J made of Cr*Au
L pole, 3...Cr+Ni, W
* Upper electrode 4 made of any one metal thin film of Au...Glass substrate 5...St substrate 6...Platinum thermometer 7...IC section 8...Electrode pad patent applicant: Nippon Telegraph and Telephone Co., Ltd. -7 and 1 other person 2nd humidity characteristics

Claims (1)

[Claims] A first ZnO layer having an electrical resistivity of 10^-^2 Ωcm or less is placed on the lower electrode formed by sequentially laminating Cr and Au.
After sequentially laminating a second ZnO layer with an electrical resistivity of 10^RΩcm or more, Cr, Ni, W, Au, Pd, P
A diode type humidity sensor characterized by laminating one type of metal among the above.