A humidity and temperature sensor based on a functional electrode layer and a preparation method thereof

Abstract

The present invention belongs to the technical field of radio frequency microelectromechanical systems, and provides a temperature and humidity sensor based on a functional electrode layer and a preparation method thereof, so as to solve the problems of large volume, low yield, weak detection signal, etc. existing in existing devices. The temperature and humidity sensor of the present invention includes: a substrate and a resonator suspended on the substrate through a support beam; a disk-shaped resonator with an etching window is adopted, and a functional electrode layer 4 and a detection electrode layer 2 are respectively arranged on the upper and lower surfaces of the piezoelectric thin film layer 3. The functional electrode is grounded, and the resonant signal is detected through the detection electrode, and temperature and humidity sensing are respectively realized through frequency information and amplitude information; the use of the functional electrode layer and the low acoustic loss layer, the separated design of the detection electrode layer and the functional electrode layer, and the etching window all effectively improve the stability of the device; moreover, the preparation method of the temperature and humidity sensor has the advantages of simple and reliable process, which is beneficial to realizing industrial production with high yield.

Description

Technical Field

[0001] The present invention belongs to the technical field of radio frequency microelectromechanical systems, relates to a temperature and humidity sensor, and specifically provides a temperature and humidity sensor based on a functional electrode layer and a preparation method thereof. Background Art

[0002] Temperature and humidity are two important physical quantities that can be experienced everywhere in life. The normal operation of nature and the survival and development of human beings are closely related to temperature and humidity. In daily life, many common health problems are closely related to temperature and humidity. The temperature of the environment where the human body is located is a key factor directly affecting human health. The environmental humidity where the human body is located is also related to many skin and respiratory diseases. Precise detection of temperature and humidity and ensuring appropriate environmental humidity can avoid the occurrence of some diseases. In addition, in industrial production such as circuit printing, textile industry, food processing industry, etc., as well as in agronomy and animal husbandry, it is also of great significance to accurately and effectively monitor and control the environmental temperature and humidity. Therefore, based on the demand for temperature and humidity testing in various fields and the development of Internet of Things technology, the research and development of temperature and humidity sensors with excellent sensing performance has always been the focus of extensive attention.

[0003] Micro-resonant sensors based on the piezoelectric effect have been widely studied and applied, mainly including thin-film bulk acoustic wave resonators (FBAR) and Lamb wave resonators (LWR); among them, FBAR and LWR mostly use rectangular piezoelectric resonators, which have excellent performance in the ultra-high frequency band (>30 MHz), but have problems such as large size, large influence of parasitic parameters on the system, and low piezoelectric material conversion efficiency in the HF band (3 - 30 MHz). On the premise of the same performance level, the size of the disk-shaped resonator is much smaller than that of the rectangular resonator, which makes the disk-shaped resonator more suitable for use as a micro-sensor; in addition, most existing temperature and humidity sensors coat functional materials on the surface of the resonator body as the functional layer, which not only makes the design and processing of the sensor very complicated, but also may cause damage to the top electrode of the device due to the coating of the functional material. Summary of the Invention

[0004] The purpose of the present invention is to provide a temperature and humidity sensor based on a functional electrode layer and a preparation method thereof, aiming at the problems of large volume, low yield rate, weak detection signal, etc. existing in existing temperature and humidity sensors, to realize temperature detection and humidity detection under the same sensor, and the device is compatible with CMOS (Complementary Metal-Oxide-Semiconductor), realizing the integration of the sensor.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A temperature and humidity sensor based on a functional electrode layer, comprising: a substrate and a resonator suspended on the substrate through a support beam; characterized in that the resonator is disc-shaped and is composed of a low acoustic loss layer 1, a detection electrode layer 2 disposed above the low acoustic loss layer, a piezoelectric thin film layer 3 disposed above the detection electrode layer, and a functional electrode layer 4 disposed on the piezoelectric thin film layer; the detection electrode layer is composed of a symmetrically arranged inferior arcuate input electrode and an inferior arcuate output electrode; a plurality of etching windows 5 are formed in the resonator, the etching windows are distributed in a circular ring array or a square array, and the centers of the arrays are all located at the center of the resonator, and the etching windows penetrate to the air cavity below the functional electrode layer.

[0007] Further, the shape of the etching window is rectangular, trapezoidal, elliptical, circular or strip-shaped with rounded arcs at both ends.

[0008] Further, the support beam is two pairs of T-beam support beams, and the two pairs of T-shaped support beams are respectively located on two mutually perpendicular center lines of the resonator.

[0009] Further, the functional electrode layer is made of graphene, silicene or molybdenum disulfide, and its thickness is 100 nanometers to 300 nanometers.

[0010] Further, the substrate is made of single crystal silicon, silicon carbide or diamond, and its thickness is 300 micrometers to 500 micrometers; the low acoustic loss layer is made of single crystal silicon or diamond, and its thickness is 5 micrometers to 10 micrometers; the detection electrode layer is made of metal molybdenum (Mo), gold (Au) or platinum (Pt), and its thickness is 100 nanometers to 300 nanometers; the piezoelectric thin film layer is made of lithium niobate (LN), lithium tantalate (LT), aluminum nitride (AlN), zinc oxide (ZnO) or lead zirconate titanate (PZT), and its thickness is 400 nanometers to 600 nanometers.

[0011] The preparation method of the above temperature and humidity sensor based on a functional electrode layer is characterized by including the following steps:

[0012] S1: Clean the substrate;

[0013] S2: Etch an air cavity in the substrate through a photolithography process and a reactive ion etching (RIE) process;

[0014] S3: Fill the air cavity with a sacrificial layer through a thin film deposition process, and planarize the upper surface of the substrate through a chemical mechanical polishing (CMP) process;

[0015] S4: Prepare a low acoustic loss layer to cover the upper surface of the device through a bonding method;

[0016] S5: Prepare a patterned detection electrode layer, metal pads, and metal connection lines on the upper surface of the low acoustic loss layer through a lift-off process and a metal thin film sputtering process. The detection electrode layer is located directly above the sacrificial layer.

[0017] S6: Prepare a piezoelectric thin film layer to cover the detection electrode layer through a thin film deposition process.

[0018] S7: Open etching windows at preset positions on the low acoustic loss layer, the electrode layer, and the piezoelectric thin film layer through a photolithography process and a reactive ion etching (RIE) process.

[0019] S8: Inject an etchant through the etching windows to remove the sacrificial layer in the air cavity.

[0020] S9: Prepare a graphene thin film, a silicene thin film, or a molybdenum disulfide thin film on a metal substrate through chemical vapor deposition (CVD), and transfer the thin film on the metal substrate to the upper surface of the piezoelectric thin film layer using a dry transfer method as a functional electrode layer, thus obtaining the humidity and temperature sensor based on a functional electrode in this embodiment.

[0021] In terms of the working principle:

[0022] In the present invention, by designing a disk-shaped resonator that is stably vibrated by four T-shaped beams, compared with the traditional rectangular interdigital resonator, the disk-shaped resonator of the present invention has better performance when operating in the frequency band of 3 - 30 MHz, with less influence from parasitic parameters. Moreover, the disk-shaped resonator in the present invention can effectively improve the electromechanical conversion efficiency of the piezoelectric material, which is more beneficial to the sensor backend detection circuit. Further, in the disk-shaped resonator, graphene is used as the functional electrode layer. As a two-dimensional crystal composed of closely packed carbon atoms, graphene has very good mechanical properties (i.e., high strength and high toughness). The high-toughness electrode material is very helpful for the stability of the device. Additionally, after graphene absorbs water, its electrical properties change, which is conducive to the backend circuit to detect the environmental humidity and achieve humidity sensing. Furthermore, in the disk-shaped resonator, the detection electrode layer (the inferior bow-shaped input electrode and the inferior bow-shaped output electrode) is embedded in the piezoelectric thin film layer without contacting the functional electrode layer, so that changes in the external environment will not affect the physical structure of the electrode, improving the stability of the physical quantity detection. In addition, by opening etching windows corresponding to the low acoustic loss layer, the electrode layer, and the piezoelectric thin film layer in the present invention, the sacrificial layer can be etched completely better, so that the resonator vibrates in the air cavity and the acoustic wave energy will not leak. This method improves the performance of the humidity and temperature sensor and increases the output signal of the sensor, facilitating the detection of the backend circuit.

[0023] The following details the humidity and temperature sensing principle based on the above disk-shaped resonator:

[0024] In the present invention, a graphene material that is electrically conductive and has humidity-sensitive properties is used as the functional electrode layer, two inferior arc-shaped metal electrodes are used as the detection electrode layer, and the piezoelectric thin film layer is located between the functional electrode layer and the detection electrode layer. When there is an electric field between the detection electrode layer and the functional electrode layer, the piezoelectric thin film layer deforms due to the inverse piezoelectric effect, and due to the piezoelectric effect, the detection electrode layer collects charges, thereby achieving the purpose of signal detection;

[0025] When the ambient temperature changes, since the low acoustic wave loss layer is made of single crystal silicon or diamond material, which is highly sensitive to temperature, the change in temperature will cause a change in the energy levels in the material. The electrons whose energy levels are raised due to strain transfer to lower energy valleys, thereby minimizing the free energy in the crystal. This change in free energy affects the elastic constant of the material, resulting in a change in the resonant frequency; by detecting the frequency change through an external circuit, temperature sensing is achieved;

[0026] When the ambient humidity changes, the graphene material has humidity-sensitive properties. After graphene absorbs water, its electrical properties will change, that is, the capacitance will change, thereby affecting the amplitude of the signal detected by the backend circuit. There is a corresponding functional relationship between the amplitude and the humidity change. By detecting the amplitude change through an external circuit, humidity sensing is achieved.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] The present invention provides a temperature and humidity sensor based on a functional electrode layer, which adopts a disc-shaped resonator. A functional electrode layer and a detection electrode layer are respectively arranged on the upper and lower surfaces of the piezoelectric thin film layer. By applying an electric field to the bottom input electrode and grounding the top functional electrode, an electric field is generated on both sides of the piezoelectric thin film layer. Due to the positive and inverse piezoelectric effects of the piezoelectric material, charges are collected at the bottom output electrode, and then the backend circuit detects and reads the signal, and temperature sensing and humidity sensing are respectively achieved through frequency information and amplitude information; graphene as the functional electrode layer can not only achieve humidity sensing but also improve the stability of the sensor, and the structure in which the detection electrode layer is embedded in the piezoelectric thin film layer and separated from the functional electrode layer also greatly reduces the influence of external environmental changes on the physical structure of the electrode, further improving the stability of the sensor; at the same time, the low acoustic wave loss layer located under the detection electrode layer can also effectively improve the stability of the sensor, avoiding the problem of easy fracture caused by only using the piezoelectric thin film layer as the resonator; the design of the etching window also enables the sacrificial layer to be etched completely better, so that the resonator vibrates in the air cavity and the acoustic wave energy does not leak. This method improves the performance of the temperature and humidity sensor, increases the output signal of the sensor, and is convenient for the detection of the backend circuit; finally, the present invention also provides a preparation method for the temperature and humidity sensor, which has the advantages of simple and reliable process, and is conducive to realizing industrial production with high yield. Description of the Drawings

[0029] Figure 1 Schematic top view of the humidity and temperature sensor based on the functional electrode layer provided by the present invention;

[0030] Figure 2 is Figure 1 Schematic cross-sectional view of the A-A' of the humidity and temperature sensor based on the functional electrode layer in

[0031] Figure 3 Schematic step-by-step process diagram of the preparation method of the humidity and temperature sensor based on the functional electrode layer provided by the present invention;

[0032] Among them, 1 is the low acoustic wave loss layer, 2 is the detection electrode layer, 3 is the piezoelectric thin film layer, 4 is the functional electrode layer, and 5 is the etching window. Detailed implementation manners

[0033] To make the objectives, technical solutions and beneficial effects of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0034] Embodiment 1

[0035] This embodiment provides a humidity and temperature sensor based on a functional electrode layer, and its structure is as shown in Figure 1 、 Figure 2 ; specifically including: a substrate and a resonator suspended on the substrate through a support beam. Among them, the resonator is disc-shaped and is composed of a low acoustic wave loss layer 1, a detection electrode layer 2 arranged above the low acoustic wave loss layer, a piezoelectric thin film layer 3 arranged above the detection electrode layer, and a functional electrode layer 4 arranged on the piezoelectric thin film layer; the detection electrode layer is composed of a symmetrically arranged inferior bow-shaped input electrode and an inferior bow-shaped output electrode, and the inferior bow-shaped input electrode and the inferior bow-shaped output electrode are respectively electrically connected to an input metal pad and an output metal pad symmetrically arranged on the substrate through metal leads arranged on the support beam; 9 square etching windows 5 are opened in the resonator, the etching windows are arranged in a square array, and the centers of the arrays are all located at the center of the resonator, and the etching windows penetrate to the air cavity below the functional electrode layer. It should be noted that: the "inferior bow-shaped" refers to: the smaller part of the two parts into which the circle is divided by the chord (straight line), and the other part is the "superior bow-shaped".

[0036] This embodiment also provides a preparation method for the above-mentioned humidity and temperature sensor based on the functional electrode layer, as shown in Figure 3 , specifically including the following steps:

[0037] S1: Use a single crystal silicon wafer with a crystal orientation of <100> as the substrate and clean it for use. Its diameter is 100 mm and its thickness is 400 μm;

[0038] S2: An air cavity is etched in a single-crystalline silicon substrate through a photolithography process and a reactive ion etching (RIE) process, as shown in Figure 3 Figure (a) therein;

[0039] S3: The air cavity is filled with polysilicon as a sacrificial layer through a thin-film deposition process, and then the polysilicon material protruding above the upper surface of the substrate is removed through a chemical mechanical polishing (CMP) process, i.e., the upper surface of the substrate is planarized, as shown in Figure 3 Figure (b) therein;

[0040] S4: A single-crystalline silicon thin film with a thickness of 5 microns is prepared to cover the upper surface of the device through a bonding method as a low acoustic wave loss layer, as shown in Figure 3 Figure (c) therein;

[0041] S5: A patterned metal molybdenum layer with a thickness of 100 nm is prepared on the upper surface of the low acoustic wave loss layer through a lift-off process and a metal thin film sputtering process as a detection electrode layer, a metal pad, and the metal wire therebetween. The detection electrode layer is located directly above the sacrificial layer, as shown in Figure 3 Figure (d) therein;

[0042] S6: A zinc oxide thin film with a thickness of 600 nm is prepared to cover the detection electrode layer through a thin-film deposition process as a piezoelectric thin film layer. The piezoelectric thin film layer completely covers the electrode layer (including the gap between the inferior bow-shaped input electrode and the inferior bow-shaped output electrode), as shown in Figure 3 Figure (e) therein;

[0043] S7: Etching windows are opened at preset positions on the low acoustic wave loss layer, the electrode layer, and the piezoelectric thin film layer through a photolithography process and a reactive ion etching (RIE) process, as shown in Figure 3 Figure (f) therein;

[0044] S8: Hydrofluoric acid solution is injected through the etching window as an etchant to remove the sacrificial layer in the air cavity, as shown in Figure 3 Figure (g) therein;

[0045] S9: A graphene thin film with a thickness of 200 nanometers is prepared on a metal iron substrate through a chemical vapor deposition (CVD) method, and the graphene thin film on the metal iron substrate is transferred to the upper surface of the piezoelectric thin film layer through a dry transfer method, i.e., the humidity and temperature sensor based on a functional electrode in this embodiment is prepared, as shown in Figure 2 Figure;

[0046] The above preparation method has the characteristics of simplicity and reliability, and can achieve a high device preparation yield.

[0047] The above are only specific embodiments of the present invention. Any feature disclosed in this specification, unless specifically described, can be replaced by other equivalent or alternative features with similar purposes; all the disclosed features, or all the steps in any method or process, except for mutually exclusive features and / or steps, can be combined in any manner.

Claims

1. A preparation method of a temperature and humidity sensor based on a functional electrode layer, the temperature and humidity sensor comprising: A substrate and a resonator suspended on the substrate via support beams; characterized in that the resonator is disk-shaped and is composed of a low acoustic wave loss layer (1), a detection electrode layer (2) disposed above the low acoustic wave loss layer, a piezoelectric thin film layer (3) disposed above the detection electrode layer, and a functional electrode layer (4) disposed on the piezoelectric thin film layer; the detection electrode layer is composed of a symmetrically arranged inferior arcuate input electrode and an inferior arcuate output electrode; a plurality of etching windows (5) are formed in the resonator, the etching windows are distributed in a circular array or a square array, and the centers of the arrays are all located at the center of the resonator, and the etching windows penetrate to the air cavity below the functional electrode layer; The temperature and humidity sensor based on the functional electrode layer is prepared by the following steps: S1: Clean the substrate; S2: Etch an air cavity in the substrate by a photolithography process and a reactive ion etching (RIE) process; S3: Fill the air cavity with a sacrificial layer by a thin film deposition process, and planarize the upper surface of the substrate by a chemical mechanical polishing (CMP) process; S4: Prepare a low acoustic wave loss layer to cover the upper surface of the device by a bonding method; S5: Prepare a patterned detection electrode layer, metal pads and metal connection lines on the upper surface of the low acoustic wave loss layer by a lift-off process and a metal thin film sputtering process, and the detection electrode layer is located directly above the sacrificial layer; S6: Prepare a piezoelectric thin film layer to cover the detection electrode layer by a thin film deposition process; S7: Open etching windows at preset positions in the low acoustic wave loss layer, the detection electrode layer and the piezoelectric thin film layer by a photolithography process and a reactive ion etching (RIE) process; S8: Inject an etchant through the etching window to remove the sacrificial layer in the air cavity; S9: Prepare a graphene thin film, a silicene thin film or a molybdenum disulfide thin film on a metal substrate by chemical vapor deposition (CVD), and transfer the thin film on the metal substrate to the upper surface of the piezoelectric thin film layer by a dry transfer method as the functional electrode layer.

2. The preparation method of the temperature and humidity sensor based on the functional electrode layer according to claim 1, characterized in that, The shape of the etching window is rectangular, trapezoidal, elliptical, circular or long strip-shaped with rounded arc chamfers at both ends.

3. The preparation method of the temperature and humidity sensor based on the functional electrode layer according to claim 1, characterized in that, The functional electrode layer is made of graphene, silicene or molybdenum disulfide, and its thickness is 100 nanometers to 300 nanometers.

4. The preparation method of the temperature and humidity sensor based on the functional electrode layer according to claim 1, characterized in that, The substrate is made of single crystal silicon, silicon carbide or diamond, and its thickness is 300 micrometers to 500 micrometers.

5. The preparation method of the temperature and humidity sensor based on the functional electrode layer according to claim 1, characterized in that, The low acoustic wave loss layer is made of single crystal silicon or diamond, and its thickness is 5 micrometers to 10 micrometers.

6. The preparation method of the temperature and humidity sensor based on the functional electrode layer according to claim 1, characterized in that, The detection electrode layer is made of metal molybdenum (Mo), gold (Au) or platinum (Pt), and its thickness is 100 nanometers to 300 nanometers.

7. The preparation method of the temperature and humidity sensor based on the functional electrode layer according to claim 1, characterized in that, The piezoelectric thin film layer is made of lithium niobate (LN), lithium tantalate (LT), aluminum nitride (AlN), zinc oxide (ZnO) or lead zirconate titanate (PZT), and its thickness is 400 nanometers to 600 nanometers.

8. The preparation method of the temperature and humidity sensor based on the functional electrode layer according to claim 1, characterized in that, The support beams are two pairs of T-shaped support beams, and the two pairs of T-shaped support beams are respectively located on two mutually perpendicular center lines of the resonator.

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