CN111721814A - Humidity sensor based on resonant desorption - Google Patents

Abstract

The invention discloses a humidity sensor based on resonant desorption, comprising: a substrate with a cavity; an insulating layer with the same cavity attached to the substrate; a first resonant electrode, covering the substrate and the insulating layer The piezoelectric functional layer is covered on the first resonant electrode and the substrate insulating layer; the second resonant electrode is attached to the piezoelectric functional layer, and is combined with the first resonant electrode and the piezoelectric functional layer as a resonant module , to achieve mechanical oscillation and dehumidification, the second resonant electrode is combined with the humidity-sensitive dielectric layer and the upper electrode as a humidity-sensing module to realize humidity-sensing measurement; the humidity-sensitive dielectric layer is vertically stacked on the resonance area formed by the resonance module to realize humidity-sensing measurement; And the upper electrode is vertically stacked on the resonance area formed by the resonance module to realize humidity sensing measurement. Compared with the traditional heating and dehumidification method, the invention avoids the problems of uneven heating and slow recovery after heating, and can quickly initialize and improve the humidity response speed.

Description

Humidity sensor based on resonant desorption

technical field

The invention relates to a MEMS humidity sensor and a preparation method thereof, in particular to a humidity sensor based on resonance type desorption compatible with CMOS technology.

Background technique

Humidity is a parameter that characterizes the water vapor content in the atmosphere, generally expressed as relative humidity (%RH), and its value represents the ratio of the water vapor pressure in the air to the saturated water vapor pressure at the same temperature. Humidity is closely related to human daily life, the growth of crops, the storage of food, the maintenance of medical equipment, the manufacture of precision electronic components, and the high-altitude operations in aerospace, so humidity monitoring is becoming more and more important.

Humidity sensor is used for humidity detection. Based on the humidity-sensitive properties of humidity-sensitive functional materials, it can physically adsorb water molecules, and convert the adsorption amount of water molecules into electrical signals to realize humidity measurement. According to the different types of converted electrical signals, it can usually be divided into capacitive, resistive, inductive, resonant, optoelectronic and so on. Among them, capacitive humidity sensor has been widely studied and applied due to its high sensitivity, low power consumption, simple structure and many other advantages.

Sensitivity and hysteresis are two important indicators that affect the performance of humidity sensors. Taking a capacitive sensor as an example, the sensitivity S refers to the degree of change in capacitance relative to the change in ambient humidity. Humidity hysteresis is defined as the phenomenon that the capacitance of the humidity sensor is inconsistent under the same humidity when the humidity is rising and falling. Generally, for the same humidity-sensitive material, the stronger the hydrophilicity, the higher the sensitivity. However, it is difficult for water molecules to desorb during the dehumidification process, which greatly increases the humidity hysteresis characteristics and brings inevitable humidity detection. delays and errors. Therefore, a humidity sensor with high sensitivity and low hysteresis has always been the research goal of many scholars.

Chinese patent CN 104634832 A designs a fast-response CMOS relative humidity sensor, which is composed of a substrate, an oxide layer, a capacitive electrode, and a humidity-sensitive medium, which is characterized by corroding the substrate and the oxide layer above it to form a cavity, making The humidity-sensitive medium between the capacitor electrodes can directly contact the air on the upper and lower sides, which is conducive to the diffusion of water molecules on the humidity-sensitive material and air water molecules, and has the advantages of fast response speed, high sensitivity, and small parasitic capacitance. However, only relying on the hydrophilicity and hydrophobicity of the material itself is difficult to meet the requirements of fast humidity response under high humidity, and the structural stability is poor.

Chinese patent CN 103698367 A discloses a heated humidity sensor, which is composed of a substrate, a lower electrode, a capacitive humidity-sensitive medium layer, an upper electrode, and a heating resistor, and is characterized in that the heating circuit surrounds the humidity-sensitive capacitor. Two heated humidity sensors are used alternately, one is used for humidity measurement during heating and dehumidification, and the other is exchanged after a working cycle. The method utilizes the principle of thermal desorption to force the desorption of water molecules. The advantage is that the humidity measurement range can reach 0%-100% RH, and it can be applied in a low temperature environment of 50°C-90°C. However, its disadvantages are high cost, high power consumption, uneven heating, and difficult to control temperature distribution. The measurement of physical quantities such as capacitance, resistance and inductance is seriously affected by temperature. High temperature heating not only introduces high power consumption, but also needs to wait until the ambient temperature is restored before measuring again. In order to improve the detection efficiency, the cost can only be increased by using multiple sensors to work alternately.

SUMMARY OF THE INVENTION

In view of this, in order to obtain a humidity sensor with high sensitivity and low hysteresis, the present invention provides a humidity sensor based on resonant desorption compatible with CMOS technology, so as to at least partially solve the above problems.

In order to achieve the above purpose, the technical solution adopted in the present invention is to provide a humidity sensor based on resonance type desorption, including:

a substrate with a cavity;

Further, the substrate is bulk silicon.

an insulating layer attached to the substrate, the insulating layer having the same cavity described above;

Further, the insulating layer is silicon nitride or silicon oxide.

Further, the cavity of the substrate and the insulating layer is disposed in the resonance region of the resonance module mentioned below.

The first resonant electrode is covered on the cavity of the substrate and the insulating layer to achieve mechanical oscillation dehumidification.

a piezoelectric functional layer, covering the first resonant electrode and the substrate insulating layer;

Further, the piezoelectric functional layer material is: ZnO, quartz, aluminum nitride, lead zirconate titanate, barium titanate or piezoelectric polymer.

The second resonant electrode is attached to the piezoelectric functional layer, and is combined with the first resonant electrode and the piezoelectric functional layer as a resonant module to realize mechanical oscillation and dehumidification;

Further, the resonance module is an acoustic wave resonator, a quartz crystal oscillator, a semiconductor material oscillator or a surface acoustic wave resonator.

The humidity-sensitive dielectric layer is vertically stacked on the resonance region formed by the resonance module;

Further, the humidity-sensitive dielectric layer completely covers the surface of the second resonant electrode and the piezoelectric functional layer;

Or the humidity-sensitive dielectric layer only covers the surface of the second resonant electrode;

Further, the material of the humidity-sensitive medium layer is: polyimide, polymethyl methacrylate and derivatives thereof or porous medium humidity-sensitive material.

and an upper electrode, which is attached to the humidity-sensitive medium layer, and the upper electrode is combined with the second resonant electrode and the humidity-sensitive medium layer as a humidity-measuring module to realize humidity-sensing measurement;

Further, the upper electrode surface has a window design, and the window pattern includes a square or a circle;

Further, the upper electrode is: capacitance, resistance, inductance, resonance or photoelectricity, thereby forming a capacitance type, resistance type, inductance type, resonance type or photoelectricity humidity sensor;

In addition, the shape of the above-mentioned resonant region is the overlapping portion of the first resonant electrode and the second resonant electrode;

The first resonant electrode, the second resonant electrode and the upper electrode are all drawn out through the pad;

The materials of the first resonant electrode, the second resonant electrode and the upper electrode are: Pt, Cu, Au, Ag or Al.

Since mechanical dehumidification is fundamentally different from traditional thermal desorption in principle, the humidity sensor based on resonance desorption proposed by the present invention has the following beneficial effects:

(1) This design uses resonant mechanical dehumidification to avoid problems such as uneven heating, long recovery time to room temperature, and uncontrollable temperature field associated with thermal dehydration, which can improve the response and recovery time of humidity;

(2) The dehumidification link and the humidity measurement link of the resonant desorption humidity sensor can be closely connected, and there is no need to use two sensors alternately, which can greatly reduce the cost and improve the detection efficiency. Only one humidity sensor needs to be detected and tested. calibration;

(3) The resonant desorption humidity sensor utilizes resonant mechanical dehumidification, which belongs to active desorption and has a wider range of temperature environments. It can be applied to low, medium and high humidity detection according to the different humidity medium layers;

(4) Resonant mechanical desorption produces mechanical vibration, and the sensor surface has a certain self-cleaning ability, so it is more suitable for harsh environments;

(5) In this design, the upper electrode and the internal window of the PI are opened to increase the air contact area, increase the humidity response sensitivity, and shorten the response time;

(6) The design is compatible with the CMOS process, and the fabrication process is simple and can be mass-produced.

Description of drawings

1 is a schematic cross-sectional view of a resonant dehumidification capacitive humidity sensor provided by an embodiment of the present invention;

2 is a schematic diagram of the overall structure of the resonant dehumidification capacitive humidity sensor in the embodiment of FIG. 1;

3 is a schematic cross-sectional view of a resonant dehumidification capacitive humidity sensor with an alternative structure according to an embodiment of the present invention;

FIG. 4 is an overall schematic diagram of the resonant dehumidification capacitive humidity sensor of the alternative structural embodiment of FIG. 3 .

In the picture:

substrate 10 silicon substrate cavity 11 insulating layer 20 insulating layer cavity 21

First resonance electrode 30 First resonance electrode pad 31 Piezoelectric functional layer 40

Second resonance electrode 50 Second resonance electrode pad 51 Humidity-sensitive dielectric layer 60

Moisture-sensitive dielectric layer window 61 Upper electrode 70 Capacitor electrode pad 71

Capacitive electrode window 72

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

An embodiment of the present invention provides a humidity sensor based on resonant desorption. The humidity sensor is composed of a multi-layer structure. For the convenience of understanding, this embodiment takes a capacitive humidity sensor as an example for further description, please refer to FIG. 1 - Figure 4, first of all, a detailed introduction to the sensor structure, including:

substrate 10 with cavity 11;

In some embodiments, the material of the substrate 10 may be bulk silicon.

In this embodiment, bulk silicon is used as the substrate 10, and the substrate 10 is used as a support frame of the overall structure of the humidity sensor, so as to complete the subsequent preparation of the humidity sensor.

The insulating layer 20 is attached to the substrate 10, and the insulating layer 20 has the same cavity as described above, as a support module for the electrical signal insulation and resonance region;

In some embodiments, the material of the insulating layer 20 may be silicon nitride or silicon oxide.

The first resonant electrode 30 covers the cavity of the substrate and the insulating layer to achieve mechanical oscillation dehumidification.

The piezoelectric functional layer 40 covers the first resonant electrode 30 and the substrate insulating layer 20, so that the first resonant electrode and a second resonant electrode 50 are insulated from each other;

In some embodiments, the piezoelectric functional layer 40 is made of ZnO, and other piezoelectric materials such as quartz, aluminum nitride, lead zirconate titanate, barium titanate, or piezoelectric polymers can also be used.

The second resonant electrode 50 is attached to the piezoelectric functional layer 40, and combined with the first resonant electrode 30 and the piezoelectric functional layer 40 as a typical thin-film bulk acoustic wave resonance module, realizes mechanical oscillation and dehumidification;

In some embodiments, the overlapping portion of the first resonant electrode 30 and the second resonant electrode 50 determines the shape of the resonant region;

The thin film bulk acoustic wave resonator module can also be other forms of resonators such as quartz crystal oscillators, semiconductor material oscillators or surface acoustic wave resonators.

In this embodiment, the shape of the resonance region corresponds to the cavities 11 and 21 of the substrate 10 and the insulating layer 20 described above.

The humidity-sensitive dielectric layer 60 is vertically stacked on the resonance region formed by the resonance module;

In some embodiments, the humidity-sensitive dielectric layer 60 completely covers the surface of the second resonant electrode 50 and the piezoelectric functional layer 40, as shown in FIG. 1 and FIG. 2;

Or the humidity-sensitive dielectric layer 60 only covers the surface of the second resonant electrode 50, as shown in FIG. 3 and FIG. 4 .

In this embodiment, the thickness of the humidity-sensitive dielectric layer 60 can be 1-6um, and 2um is selected here; the material can be polyimide, or polymers such as polymethyl methacrylate and its derivatives, or porous Dielectric moisture-sensitive materials such as porous ceramic base materials, porous metal oxides and other porous semiconductor materials. The moisture-sensitive dielectric layer 60 can also be designed with a window opening (61 in FIG. 3 ).

and the upper electrode 70, which is attached to the humidity-sensitive medium layer, and the upper electrode 70 is combined with the second resonant electrode 50 and the humidity-sensitive medium layer 60 as a humidity measuring module to realize humidity-sensing measurement;

In some embodiments, the surface of the upper electrode 70 is designed with a window, and the window pattern includes a square or a circle. In this embodiment, a 10×10um square window (72 in FIG. 3 ) is selected, which is the same as the window of the above-mentioned humidity-sensitive dielectric layer 61 . match;

In this embodiment, the upper electrode is a capacitor, and the capacitive humidity sensor that can quickly resonate and dehumidify based on this design scheme is only an implementation. Photoelectric and other measurement methods are made into various humidity sensors that can quickly resonate and dehumidify.

In addition, the above-mentioned first resonant electrode 30 is drawn out through the pad 31;

The above-mentioned second resonant electrode 50 is attached on the ZnO layer, and is drawn out through the pad 51;

The upper electrode 70 is led out through the pad 71;

In this embodiment, all electrodes such as the first resonant electrode 30 , the second resonant electrode 50 and the upper electrode 70 can be made of metal Pt material, or other metal materials such as Cu, Au, Ag, and Al.

The above-mentioned humidity sensor based on resonant desorption provided by the present invention works as follows: a humidity-sensitive capacitive sensor based on a polyimide moisture-sensing functional layer, when water molecules are adsorbed on the surface of polyimide, due to the water under different humidity Different molecular adsorption concentrations lead to the change of the dielectric constant of the capacitive sensor dielectric layer and the capacitance change. Since the moisture-sensing medium layer itself may have hydrophilic groups such as porous and polyhydroxyl groups, it is easy to absorb water, so the adsorption of water molecules is easy and the desorption is difficult. The desorption module based on Thin Film Bulk Acoustic Resonator (FBAR), the FBAR can oscillate through the peripheral oscillator circuit and maintain constant amplitude oscillation. Its oscillation frequency mainly depends on the thickness of the resonance region, and the oscillation amplitude mainly depends on the driving voltage strength. When the voltage intensity is appropriate, the FBAR can resonate in the vicinity of its fundamental frequency to a large extent, and convert its mechanical energy into the kinetic energy and internal energy of the surface water molecules, so that the water molecules can be mechanically desorbed quickly.

In practical applications, a complete detection cycle of the resonant dehumidification capacitive humidity sensor can be divided into two parts: dehumidification recovery and humidity detection. When the resonance module is switched to the humidity measurement process, it is used as a capacitive humidity measurement module. According to the actual humidity environment, the time ratio and loss power of the two links can be reasonably configured.

Another embodiment of the present invention provides a preparation method of the above-mentioned resonant desorption-based humidity sensor. The preparation method of the present invention is also described in detail below with reference to FIGS. 1 to 4:

(1) Thin Film Bulk Acoustic Resonator

A 300 nm silicon nitride or silicon oxide insulating layer 20 is prepared on the silicon substrate 10 by chemical vapor deposition (CVD) or low pressure chemical vapor deposition (LPCVD).

(2) Preparation of FBAR lower electrode

The 200nm/30nm thick Pt/Ti is sputtered on the surface of the insulating layer 20 on silicon by photolithography and sputtering, and the lower electrode of the FBAR, ie, the first resonant electrode 30, is formed by a Lift-off process.

(3) Preparation of FBAR piezoelectric functional layer ZnO

A layer of zinc oxide with a thickness of 2 μm is sputtered on the surface of the lower electrode of the FBAR, that is, the first resonant electrode 30 by a radio frequency magnetron sputtering process, and a photoresist mask layer with a fixed pattern is formed by photolithography. The ZnO layer was patterned to prepare the piezoelectric functional layer 40 .

(4) Preparation of FBAR upper electrode

The 200nm/30nm thick Pt/Ti is sputtered on the surface of the ZnO layer by photolithography and sputtering, and the FBAR upper electrode ie the second resonant electrode 50 is formed by the Lift-off process, which also serves as the lower electrode of the PI capacitive sensor.

(5) Preparation of PI layer

The polyamic acid is uniformly coated on the surface of the upper electrode of the FBAR, that is, the second resonant electrode 50 by a sol-gel method, and is fixed to form a film by soft baking, and a photoresist mask layer with a fixed pattern is formed by photolithography, and plasma etching is used to form a photoresist mask layer with a fixed pattern. The dielectric layer is patterned by means of (ICP), and finally, the humidity-sensitive dielectric layer 60 is prepared by imidization at a high temperature of 300°C.

(6) Preparation of upper electrode of PI capacitor

Pt/Ti with a thickness of 200 nm/30 nm is sputtered on the surface of the PI layer by a photolithography process and a sputtering process, and a PI capacitor upper electrode 70 is formed by a Lift-off process.

(7) Si substrate cavity release and chip separation

The thick glue mask layer of window and scribe groove pattern is formed by photolithography, and the back insulating layer of Si is etched by trifluoromethane dry method by means of ICP. By means of deep ion etching DRIE, the Si substrate 10 is dry-etched by sulfur hexafluoride to form a cavity 11, and finally, by means of ICP, the insulating layer 20 on the front side of Si is dry-etched by trifluoromethane to form a cavity 11. For the cavity 21, for example, the area of the cavities 11 and 21 can be selected as 500×500um. . Finally, the release of the cavity and the self-separation of the chip are realized.

So far, the preparation of the resonant desorption humidity sensor is completed.

The humidity sensor based on resonant desorption provided by the invention is different from the principle of thermal desorption, and can not only retain the high hydrophilicity and high sensitivity of the humidity sensitive material itself, but also does not need to greatly change the temperature of the sensor and prolong the recovery time. Compared with the heated humidity sensor, it has the characteristics of low power consumption, long-term stability, high sensitivity, fast response speed, wide operating temperature range and high humidity range. It is suitable for complex and changeable high-altitude meteorological environment, and is conducive to high-frequency humidity detection.

The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above-mentioned specific embodiments are only specific embodiments of the present invention, and are not intended to limit the present invention. Within the spirit and principle of the present invention, any modifications, equivalent replacements, improvements, etc. made should be included within the protection scope of the present invention.

Claims (10)

1. a kind of humidity sensor based on resonance type desorption, is characterized in that, comprises: a substrate with a cavity; an insulating layer attached to the substrate, the insulating layer having the same cavity; The first resonant electrode is covered on the cavity of the substrate and the insulating layer to realize mechanical oscillation and dehumidification; a piezoelectric functional layer, covering the first resonant electrode and the substrate insulating layer; The second resonant electrode is attached to the piezoelectric functional layer, and is combined with the first resonant electrode and the piezoelectric functional layer as a resonant module to achieve mechanical oscillation dehumidification; The humidity-sensitive dielectric layer is vertically stacked on the resonance region formed by the resonance module; and an upper electrode, which is attached to the humidity-sensitive medium layer, and the upper electrode is combined with the second resonant electrode and the humidity-sensitive medium layer as a humidity-measuring module to realize humidity-sensing measurement. 2 . The humidity sensor based on resonant desorption according to claim 1 , wherein the first resonant electrode, the second resonant electrode and the upper electrode are all drawn out through a pad. 3 . 3 . The humidity sensor based on resonance desorption according to claim 1 , wherein the shape of the resonance region is the overlapping portion of the first resonance electrode and the second resonance electrode. 4 . 4 . The humidity sensor based on resonance desorption according to claim 1 , wherein the resonance module is an acoustic wave resonator, a quartz crystal oscillator, a semiconductor material oscillator or a surface acoustic wave resonator. 5 . 5 . The humidity sensor based on resonant desorption according to claim 1 , wherein the humidity-sensitive dielectric layer completely covers the surface of the second resonant electrode and the piezoelectric functional layer. 6 . 6 . The humidity sensor based on resonant desorption according to claim 1 , wherein the humidity-sensitive dielectric layer covers the surface of the second resonant electrode. 7 . 7 . The humidity sensor based on resonant desorption according to claim 1 , wherein the upper electrode surface has a window design, and the window pattern includes a square or a circle. 8 . 8 . The humidity sensor based on resonance type desorption according to claim 1 , wherein the upper electrode is: capacitance, resistance, inductance, resonance or photoelectric, forming capacitance type, resistance type, inductance type, resonance type Or photoelectric humidity sensor. 9 . The humidity sensor based on resonance desorption according to claim 1 , wherein the cavity of the substrate and the insulating layer is disposed in the resonance region of the resonance module. 10 . 10. The humidity sensor based on resonant desorption according to claim 1, wherein: the substrate is bulk silicon; The insulating layer is silicon nitride or silicon oxide; The materials of the first resonant electrode, the second resonant electrode and the upper electrode are: Pt, Cu, Au, Ag or Al; The piezoelectric functional layer material is: ZnO, quartz, aluminum nitride, lead zirconate titanate, barium titanate or piezoelectric polymer; The material of the humidity-sensitive medium layer is: polyimide, polymethyl methacrylate and derivatives thereof or porous medium humidity-sensitive material.

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