CN107290241B - A kind of QCM humidity sensor and preparation method thereof - Google Patents

Abstract

The invention discloses a QCM humidity sensor, which is characterized in that: the piezoelectric ceramic comprises a piezoelectric wafer, an insulating layer, a metal electrode layer and a humidity sensitive film layer; the metal electrode layer comprises a first metal electrode layer and a second metal electrode layer; the first metal electrode layer comprises two electrodes which are respectively arranged on two surfaces of the piezoelectric wafer, and an insulating layer is arranged on one surface of the electrode; the surface of the insulating layer is provided with a second metal electrode layer, and the surface of the second metal electrode layer is provided with a humidity sensitive film layer. The QCM humidity sensor has the structure of the bimetal electrode layer, can achieve high-sensitivity detection of low humidity, can enhance the stability of the sensor, has the advantages of low cost, large humidity detection range, high sensitivity, quick response, good repeatability and simple manufacturing process, and can be widely applied to the fields of low humidity detection and high-precision humidity detection.

Description

A kind of QCM humidity sensor and preparation method thereof

technical field

The invention relates to a humidity sensor, in particular to a QCM humidity sensor and a preparation method thereof.

Background technique

As a kind of important sensor, humidity sensor has a very wide range of applications in meteorology, agriculture, industrial control and other fields. With the continuous development of science and technology, people's demand for high-performance humidity sensors continues to increase. Traditional hair humidity sensors and psychrometer humidity sensors have long been unable to meet the requirements of the industry, so various types of electronic humidity sensors have emerged. In the 1960s, a quartz crystal microbalance (QCM) type humidity sensor appeared.

其中f₀为压电晶片的固有基频(单位为Hz)，A为金属电极面积(单位为cm²)，ρ_q、μ_q分别为压电晶片的密度和剪切模量。由于QCM传感器具有很高的灵敏度、较低成本低廉，而且测试装置简单、易于实现现场连续检测等众多优点，使得QCM湿度传感器广泛应用于湿度检测领域。The QCM humidity sensor is composed of a quartz crystal resonator and a humidity-sensitive thin film layer modified on the surface of the resonator; the humidity-sensitive thin film deposited on the metal electrode absorbs water molecules and causes the mass of the sensitive thin film to increase (Δm), thereby generating a molecular weight that is consistent with the molecular weight of the adsorbed water The relevant frequency offset (Δfm) is related to the humidity to be measured in the environment due to the amount of water molecule adsorption, and the humidity in the environment can be obtained by measuring the resonance frequency offset; the working principle of the QCM humidity sensor is based on the following relationship:

Where f ₀ is the inherent fundamental frequency of the piezoelectric wafer (in Hz), A is the area of the metal electrode (in cm ² ), ρ _q and μ _q are the density and shear modulus of the piezoelectric wafer, respectively. Because the QCM sensor has many advantages such as high sensitivity, low cost, simple test device, and easy on-site continuous detection, the QCM humidity sensor is widely used in the field of humidity detection.

The detection sensitivity of existing QCM humidity sensors is usually not high in the low humidity range (<10%RH), because the content of water molecules in the air in this humidity range is very low, which directly leads to adsorption to the humidity sensitive film layer. The number of water molecules on the surface is also very small, so the frequency offset of the QCM humidity sensor is also small. In order to increase the detection sensitivity of the QCM humidity sensor in the low humidity range, the sensitization methods that the industry has adopted include: (1) using a humidity-sensitive material with a nanostructure to increase the specific surface area of the material; (2) using a composite humidity-sensitive material To enhance the hydrophilic properties of the material; (3) Use a large dose of moisture-sensitive materials to increase the adsorption points of water molecules. However, the sensitivity enhancement methods of the above three QCM humidity sensors in the low humidity range are all based on the premise that the humidity sensitive material absorbs more water molecules. The adsorption of a large number of water molecules will greatly increase the viscosity of the humidity-sensitive film deposited on the metal electrode of the QCM humidity sensor, resulting in abnormal fluctuations in the output frequency of the QCM humidity sensor, vibration stop and other abnormal phenomena, and the stability of the sensor is greatly reduced. This is extremely detrimental to the detection accuracy of the sensor.

Contents of the invention

The object of the invention of the present invention is to: address the above-mentioned problems, by improving the structure of the existing QCM humidity sensor, a double metal electrode layer structure is proposed, and a layer of carbon dioxide is sputtered on the surface of any metal electrode of the QCM humidity sensor. Silicon film, and then form interdigitated electrodes on the surface of the silicon dioxide film by evaporation metal process, photolithography process and etching process, and deposit a layer of water-absorbing electrode on the surface of the interdigitated electrode by air spraying, spin coating, drop coating, etc. A medium humidity sensitive film; a QCM humidity sensor is prepared.

The technical scheme that the present invention adopts is as follows:

A kind of QCM humidity sensor of the present invention comprises piezoelectric chip, insulating layer, metal electrode layer and humidity sensitive film layer; Described metal electrode layer comprises first metal electrode layer and second metal electrode layer; Described first metal electrode layer It includes two electrodes, which are respectively arranged on the two surfaces of the piezoelectric wafer, and an insulating layer is arranged on the surface of one electrode; a second metal electrode layer is arranged on the surface of the insulating layer, and a humidity sensitive film is arranged on the surface of the second metal electrode layer layer.

Further, the piezoelectric wafer is a piezoelectric quartz wafer, which is AT-cut.

Further, the fundamental frequency of the piezoelectric quartz wafer is 5-20 MHz.

Further, the material of the metal electrode layer is gold or silver.

Further, the second metal electrode layer has an interdigitated electrode structure.

Further, the interdigitated electrodes are respectively connected to pin three and pin four; the first metal electrode layer includes metal electrode one and metal electrode two; metal electrode two is arranged between the piezoelectric chip and the insulating layer, and the metal electrode The second is connected to pin two; the metal electrode one is connected to pin one, and is in contact with the piezoelectric chip.

Further, the humidity-sensitive film layer is a water-absorbing medium material with a dielectric constant less than 5.

Further, the humidity sensitive thin film layer is formed on the surface of the second metal electrode layer by air spraying, spin coating or drop coating, and part of the sensitive thin film layer is filled in the electrode gap of the second metal electrode layer.

Its preparation method of a kind of QCM humidity sensor of the present invention comprises the following steps:

S1: Forming a first metal electrode layer on both surfaces of the piezoelectric quartz wafer through an evaporation process;

S2: forming a 50nm thick silicon dioxide insulating layer on the surface of the first metal electrode layer by a sputtering process;

S3: forming a second metal electrode layer on the surface of the silicon dioxide insulating layer by an evaporation process;

S4: modifying the second metal electrode layer by a photolithography process and an etching process to form an interdigitated electrode structure;

S5: Washing with alcohol and deionized water, the piezoelectric quartz wafer with metal electrodes processed above to remove contaminants, and drying;

S6: forming a humidity-sensitive thin film layer on the surface of the second metal electrode layer by air spraying, spin coating or drop coating, and drying.

In the QCM humidity sensor of the present invention, pin two is connected to pin three, and pin one and pin four are used as two output terminals of the sensor to connect to two pins of an external oscillation circuit. Under the working condition of this pin configuration, The QCM humidity sensor and the interdigitated humidity-sensitive capacitor share a humidity-sensitive film layer, so the humidity sensor device can be regarded as a QCM humidity sensor and an interdigitated humidity-sensitive capacitor connected in series, and its equivalent circuit diagram is shown in Figure 3; In the equivalent circuit, the component resistance (R), inductance (L) and capacitance (C) respectively reflect the mechanical loss, inertial mass and crystal elasticity of the QCM, and the capacitance (C ₀ ) reflects the intrinsic capacitance of the QCM; among them, the humidity of the QCM Adsorption of water molecules by the sensitive film layer of the sensor will increase the inertial mass (that is, the L parameter) of the QCM, thereby generating a frequency shift Δf _m related to the mass of the adsorbed water molecules;

The equivalent humidity-sensitive capacitance C _X reflects the edge-type humidity-sensitive capacitance formed by electrodes 3 and 4 as fringe capacitance electrodes, and the humidity-sensitive film layer as the filling medium. When the sensitive film absorbs water molecules, its dielectric constant will increase. Large, so that C _X increases, which causes the sensor to produce a frequency shift Δf _ε related to the dielectric constant of the sensitive film layer; Δf = Δf _m + Δf _ε , and both Δf _m and Δf _ε are negative due to water adsorption, so a superposition effect is generated to increase the low humidity response sensitivity.

In summary, owing to adopting above-mentioned technical scheme, the beneficial effect of the present invention is:

1. The QCM humidity sensor prepared by the present invention introduces the structure of the bimetallic electrode layer, and is sensitive to the quality change and the dielectric constant change of the humidity-sensitive film layer simultaneously, and the quality and the dielectric constant of the humidity-sensitive film layer after absorbing water molecules The change will make the QCM humidity sensor produce frequency response sensitivity, so that the purpose of detecting low humidity with high sensitivity can be achieved under the condition of a small amount of adsorbed water molecules; The stability of the sensor decreases, so the stability of the sensor can be enhanced.

2. The QCM humidity sensor prepared by the present invention has low cost, large humidity detection range (0.1-100% RH), high sensitivity, fast response, good repeatability, simple manufacturing process, and can be widely used in low humidity detection and high humidity detection. precision humidity detection field.

Description of drawings

The invention will be illustrated by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a schematic structural diagram of a QCM humidity sensor of the present invention.

Fig. 2 is a top view of a QCM humidity sensor of the present invention.

Fig. 3 is an equivalent circuit diagram of a QCM humidity sensor of the present invention.

Marks in the figure: 1 is the piezoelectric wafer, 2 is the insulating layer, 3 is the metal electrode layer, 31 is the first metal electrode layer, 32 is the second metal electrode layer, and 4 is the humidity sensitive film layer.

Detailed ways

All features disclosed in this specification, or steps in all methods or processes disclosed, may be combined in any manner, except for mutually exclusive features and/or steps.

Any feature disclosed in this specification (including any appended claims, abstract), unless otherwise stated, may be replaced by alternative features which are equivalent or serve a similar purpose. That is, unless expressly stated otherwise, each feature is one example only of a series of equivalent or similar features.

Embodiment one:

As in Fig. 1, comprise piezoelectric wafer 1, insulation layer 2, metal electrode layer 3 and humidity sensitive film layer 4; Described metal electrode layer 3 comprises first metal electrode layer 31 and second metal electrode layer 32; The first Metal electrode layer 31 comprises two electrodes, is arranged on two faces of piezoelectric wafer 1 respectively, wherein one electrode surface is provided with insulating layer 2; Said insulating layer 2 surface is provided with second metal electrode layer 32, and the second metal The surface of the electrode layer 32 is provided with a humidity sensitive film layer 4 .

Embodiment two:

Compared with Embodiment 1, this embodiment also discloses that the piezoelectric wafer 1 is a piezoelectric quartz wafer, which is AT-cut; the fundamental frequency of the piezoelectric quartz wafer is 5-20 MHz.

Embodiment three:

Compared with the previous embodiments, this embodiment also discloses that the material of the metal electrode layer 3 is gold or silver.

Embodiment four:

As shown in FIG. 2 , compared with the previous embodiments, this embodiment also discloses that the second metal electrode layer 32 has an interdigitated electrode structure.

The interdigitated electrodes are respectively connected to pin three 323 and pin four 324; the first metal electrode layer 31 includes metal electrode one and metal electrode two; metal electrode two is arranged between the piezoelectric wafer 1 and the insulating layer 2 , the second metal electrode is connected to the second pin 312 ; the first metal electrode is connected to the first pin 311 , and is in contact with the piezoelectric wafer 1 .

Embodiment five:

Compared with the previous embodiments, this embodiment also discloses that the humidity-sensitive film layer 4 is a water-absorbing dielectric material with a dielectric constant less than 5.

The humidity sensitive thin film layer 4 is formed on the surface of the second metal electrode layer 32 by air spraying, spin coating or drop coating, and part of the sensitive thin film layer is filled in the electrode gap of the second metal electrode layer 32 .

Embodiment six:

Compared with the previous embodiments, this embodiment also discloses a method for preparing a QCM humidity sensor, including the following steps:

S1: Forming the first metal electrode layer 31 on both surfaces of the piezoelectric quartz wafer by evaporation process;

S2: forming a silicon dioxide insulating layer 2 with a thickness of 50 nm on the surface of the first metal electrode layer 31 by a sputtering process;

S3: forming a second metal electrode layer 32 on the surface of the silicon dioxide insulating layer 2 by an evaporation process;

S4: modifying the second metal electrode layer 32 through a photolithography process and an etching process to form an interdigitated electrode structure;

S5: Washing with alcohol and deionized water, the piezoelectric quartz wafer with metal electrodes processed above to remove contaminants, and drying;

S6: Forming the humidity sensitive thin film layer 4 on the surface of the second metal electrode layer 32 by air spraying, spin coating or drop coating method, and drying.

The operating principle of a QCM humidity sensor of the present invention is as follows: the sensor is located in the detection environment, and the moisture-sensitive film layer absorbs water molecules in the environment, thereby causing two changes in the humidity-sensitive film layer:

(1) The humidity-sensitive film layer absorbs water molecules to increase its mass. According to the mass-frequency relationship, the QCM humidity sensor will produce a frequency offset Δf _m related to the mass of adsorbed water molecules;

(2) The dielectric constant of the humidity-sensitive film layer increases due to the adsorption of water molecules, which leads to an increase in the interdigitated humidity-sensitive capacitance C _X formed between pins 3 and 4, so that the sensor will generate a The frequency offset Δf _ε related to the dielectric constant of the sensitive film layer; due to the introduction of the frequency offset Δf _ε related to the dielectric constant of the sensitive film layer, this makes the detection sensitivity of the sensor in the low humidity range (<10%RH) can be At the same time, since the change in the dielectric constant of the humidity-sensitive film layer usually does not reduce the frequency stability of the QCM sensor, it is very easy to achieve the purpose of detecting low humidity with high sensitivity and high precision.

The present invention is not limited to the foregoing specific embodiments. The present invention extends to any new feature or any new combination disclosed in this specification, and any new method or process step or any new combination disclosed.

Claims (7)

1. A QCM humidity sensor, characterized in that: comprises a piezoelectric wafer (1), an insulating layer (2), a metal electrode layer (3) and a humidity sensitive film layer (4); the metal electrode layer (3) comprises a first metal electrode layer (31) and a second metal electrode layer (32); the first metal electrode layer (31) comprises two electrodes which are respectively arranged on two surfaces of the piezoelectric wafer (1), and an insulating layer (2) is arranged on one surface of the electrode; a second metal electrode layer (32) is arranged on the surface of the insulating layer (2), and a humidity sensitive film layer (4) is arranged on the surface of the second metal electrode layer (32);

a method for preparing a QCM humidity sensor, comprising the steps of:

s1: forming first metal electrode layers (31) on both surfaces of a piezoelectric quartz wafer by an evaporation process;

s2: forming a silicon dioxide insulating layer (2) with the thickness of 50nm on the surface of the first metal electrode layer (31) through a sputtering process;

s3: forming a second metal electrode layer (32) on the surface of the silicon dioxide insulating layer (2) through an evaporation process;

s4: modifying the second metal electrode layer (32) through a photoetching process and an etching process to form an interdigital electrode structure;

s5: cleaning with alcohol and deionized water, removing pollutants from the processed piezoelectric quartz wafer with the metal electrode, and drying;

s6: and forming a humidity sensitive film layer (4) on the surface of the second metal electrode layer (32) by an air spraying, spin coating or dripping method, and drying.

2. The QCM humidity sensor of claim 1 wherein: the piezoelectric wafer (1) is a piezoelectric quartz wafer and is an AT cut.

3. The QCM humidity sensor of claim 2 wherein: the fundamental frequency of the piezoelectric quartz wafer is 5-20 MHz.

4. The QCM humidity sensor of claim 1 wherein: the metal electrode layer (3) is made of gold or silver.

5. The QCM humidity sensor of claim 1 wherein: the second metal electrode layer (32) is an interdigital electrode structure.

6. The QCM humidity sensor of claim 5 wherein: the interdigital electrode is respectively connected with a pin III (323) and a pin IV (324); the first metal electrode layer (31) comprises a first metal electrode and a second metal electrode; the second metal electrode is arranged between the piezoelectric wafer (1) and the insulating layer (2), and is connected with the second pin (312); a first metal electrode and a first connection pin (311) are contacted with the piezoelectric chip (1).

7. The QCM humidity sensor of claim 1 wherein: the humidity sensitive film layer (4) is made of a water-absorbing medium material, and the dielectric constant is smaller than 5.

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