An annular suspended MEMS device for in-situ thermal conductivity testing and a method of manufacturing the same

Abstract

This invention discloses an in-situ suspended MEMS device for thermal conductivity testing and its fabrication method. The device structure includes a silicon substrate with a hollow central section. A ring-shaped suspended platform structure serves as a structural support film and also as an electrically and thermally insulating substrate for the polycrystalline silicon layer. On the two ring-shaped suspended platforms are a first thermopile temperature sensor, a second thermopile temperature sensor, a first metal heating electrode, a second metal heating electrode, a first pyroelectric thin film, and a second pyroelectric thin film. The first thermopile temperature sensor, the first metal heating electrode, and the first pyroelectric thin film are all located on the first ring-shaped suspended platform. The second thermopile temperature sensor, the second metal heating electrode, and the second pyroelectric thin film are all located on the second ring-shaped suspended platform. An rGO (reduced graphene oxide) thermal rectifier layer is designed within a portion of the silicon nitride to increase the temperature difference across the thermopile and improve measurement accuracy.

Description

TECHNICAL FIELD

[0001] The application relates to an in-situ testing device for low-dimensional materials and a preparation method thereof, in particular to a ring-shaped suspended MEMS device suitable for in-situ steady and transient thermal conductivity testing of a scanning electron microscope and a preparation method thereof. BACKGROUND

[0002] With the reduction of device size and the increase of integrated circuit density, the heat generated per unit area of the device during the processing and operation of the electronic device is increased, which can cause many adverse effects, so it is of great significance to study the thermal properties of low-dimensional materials. Accurate measurement of the thermal conductivity of single nanowire, nanotube and suspended two-dimensional material is the basis for studying the thermal parameter characteristics of low-dimensional materials. However, there are great technical difficulties in measuring micro-nano scale temperature, effectively controlling heat flow direction and accurately measuring thermal conductivity. In 2001, Majumdar et al. of Berkeley University developed a suspended micro-bridge method to successfully measure the thermal conductivity of single multi-walled carbon nanotube. In the following decades, researchers successfully measured the thermal conductivity of single carbon nanotube, nanowire linear efficiency, silicon nanowire thermoelectric material, and made basic phonon devices such as thermal diode and thermal memory using the platform. Later, the Chen Renkun group of the University of San Diego developed a Wheatstone bridge method to measure the resistance of the sensor, which improved the measurement accuracy of the micro-bridge method. s The measurement method of the sensor R h and T s , which ignores the temperature gradient inside the heater and the sensor. In addition, the accuracy of the measuring instrument is limited. If low thermal conductivity samples are to be measured, the method must be improved to improve the measurement accuracy. Compared with the resistance micro-bridge method, the thermocouple micro-bridge method has the advantages of no temperature gradient problem, large output electromotive force and high measurement accuracy, which is of great significance to thermal conductivity measurement. SUMMARY

[0003] Purpose of the Invention: The purpose of this invention is to solve the technical difficulties in high-precision measurement of in-situ thermal conductivity of nanomaterials, making it suitable for scanning in-situ thermal parameter testing. This invention provides a ring-shaped suspended MEMS device for in-situ thermal conductivity testing and its fabrication method. The test structure in this embodiment includes a silicon substrate and a silicon nitride support film. The silicon substrate has a hollow structure in the middle, with a ring-shaped suspended platform structure serving as a structural support film and also as an electrically and thermally insulating substrate for the polycrystalline silicon layer. On the two ring-shaped suspended platforms are a first thermocouple stack temperature sensor, a second thermocouple stack temperature sensor, a first metal heating electrode, a second metal heating electrode, a first pyroelectric thin film, and a second pyroelectric thin film, with the left and right sides completely symmetrical. Each pair of thermocouples is composed of p-type / n-type polycrystalline silicon material, consisting of an upper thermocouple portion (n-type polycrystalline silicon) and a lower thermocouple portion (p-type polycrystalline silicon) stacked in a three-dimensional space, with silicon nitride used for electrical isolation between the two thermocouple layers to prevent short circuits. A silicon nitride-based rGO (reduced graphene oxide) thermal rectifier layer is designed in the middle to increase the temperature difference between the two ends of the thermopile and improve measurement accuracy. Each pair of thermopile units is led out with a pair of pads for measuring the thermopile's voltage output. The lower surface of the pyroelectric film is connected to a p-type polysilicon strip, and the upper surface is connected to an n-type polysilicon strip. Pads are led out outside the suspended platform for measuring the pyroelectric current. After the thermopile and pyroelectric structure are completed, a silicon nitride film is deposited as a structural protective layer. Finally, a metal heating electrode is deposited on the suspended platform encased in silicon nitride as a heat source.

[0004] During testing, the sample material is transferred between the second and first annular suspended platforms, with both ends fixed at arbitrary positions on the two platforms. Thermopile temperature sensors and pyroelectric transient temperature sensors are located nearby to ensure that the sample temperature is closer to the test temperature and guarantee the accuracy of the measurement.

[0005] Beneficial effects

[0006] In terms of chip structure and performance: This invention significantly improves measurement accuracy, reduces the precision requirements of measuring instruments, and supports steady-state / transient thermal conductivity measurements. The reasons and innovations are as follows: First, this invention uses a series output of thermopile thermocouples, which increases the output electromotive force compared to single-pair thermocouple temperature measurement, reducing the precision requirements of measuring instruments and saving costs while improving test accuracy. Second, the thermocouple structure in the test chip design of this invention adopts a stacked ring arrangement, which saves chip area and allows for an increase in the number of thermopile pairs within a fixed space, increasing sensor sensitivity. Furthermore, the ring arrangement solves the problem of the sample length being fixed with the spacing of the suspended platform in the traditional thermal bridge method. Theoretically, the sample length range can range from the minimum spacing to the diameter of a hollow circle, increasing sample flexibility. The traditional thermal bridge method based on resistance temperature detectors (RTDs), when designed as a ring, causes errors when measuring samples of different lengths due to different sensing points. Accurate measurement requires multiple pairs of thermal bridges, which significantly increases the area compared to a ring-shaped thermopile. The sensing point of the toroidal thermopile structure is the hot end of the thermopile structure. The end of the sample under test is close to the hot end of any adjacent thermopile. Compared with the classic suspended thermal bridge method based on resistance temperature measurement, this reduces the error caused by temperature gradient and can effectively improve measurement accuracy. Third, embedding a pyroelectric thin film on the outermost side of the suspended platform, in conjunction with the thermopile temperature sensor, can further perform steady-state / transient temperature measurements, thus filling the gap in current research on transient thermal conductivity measurement and making temperature measurements more accurate. Fourth, designing an rGO redox graphene interlayer in the fan-shaped silicon nitride cantilever, utilizing the thermal rectification principle caused by size asymmetry, controls the direction of heat flow. Heat transfer from the hot end of the thermopile to the cold end of the heat sink in the toroidal suspended platform is more difficult and concentrated at the hot end, thus increasing the temperature difference between the two ends of the thermopile and improving measurement accuracy.

[0007] In terms of chip functionality: This chip can be used for high-precision in-situ steady-state and transient thermal conductivity testing in scanning electron microscopy. It also supports testing samples with length differences, making it easy to obtain the microstructure-property relationship between sample structure, size and thermal parameters.

[0008] Regarding the requirements for testing equipment: Since the use of thermopile increases the output electromotive force, the accuracy requirements of high-precision instruments for testing small signals are reduced, or circuit modules can be used directly for amplification and measurement, which is inexpensive and more portable. Attached Figure Description

[0009] Figure 1 A schematic diagram of a ring-shaped suspended MEMS device for testing stable and transient thermal conductivity.

[0010] Figure 2 A schematic diagram of the second core part of the toroidal suspended MEMS device used for stable and transient thermal conductivity testing;

[0011] Figure 3This is a structural cross-sectional view of the first step in testing the stable and transient thermal conductivity of a toroidal suspended MEMS device.

[0012] Figure 4 This is a structural cross-sectional view of the second step in testing the steady-state and transient thermal conductivity of a toroidal suspended MEMS device.

[0013] Figure 5 This is a structural cross-sectional view of the third step in testing the steady-state and transient thermal conductivity of a toroidal suspended MEMS device.

[0014] Figure 6 This is a structural cross-sectional view of the fourth step in testing the steady-state and transient thermal conductivity of a toroidal suspended MEMS device.

[0015] Figure 7 This is a structural cross-sectional view of the fifth step in testing the steady-state and transient thermal conductivity of a toroidal suspended MEMS device.

[0016] Figure 8 This is a structural cross-sectional view of the sixth step in testing the steady-state and transient thermal conductivity of a toroidal suspended MEMS device.

[0017] Figure 9 This is the structural cross-sectional view of the seventh step in testing the stable and transient thermal conductivity of a toroidal suspended MEMS device.

[0018] Figure 10 This is the structural cross-sectional view of the eighth step in testing the steady-state and transient thermal conductivity of a toroidal suspended MEMS device.

[0019] Figure 11 This is the structural cross-sectional view of the ninth step in testing the stable and transient thermal conductivity of a toroidal suspended MEMS device.

[0020] Figure 12 This is the tenth structural cross-sectional view of a toroidal suspended MEMS device used for stable and transient thermal conductivity testing.

[0021] Figure 13 This is the eleventh step of the structural cross-sectional view of the toroidal suspended MEMS device for stable and transient thermal conductivity testing;

[0022] Figure 14 This is a structural cross-sectional view of the twelfth step in testing the steady-state and transient thermal conductivity of a toroidal suspended MEMS device.

[0023] Figure 15 This is a structural cross-sectional view of the thirteenth step in testing the steady-state and transient thermal conductivity of a toroidal suspended MEMS device.

[0024] Figure 16 This is a structural cross-sectional view of the fourteenth step in testing the steady-state and transient thermal conductivity of a toroidal suspended MEMS device.

[0025] Figure 17 This is a temperature simulation diagram of a classic MEMS thermal bridge structure;

[0026] Figure 18 This is a temperature simulation diagram of a toroidal suspended MEMS device used for stable and transient thermal conductivity testing.

[0027] Figure 19 This is a potential simulation diagram of a toroidal suspended MEMS device used for steady-state and transient thermal conductivity testing.

[0028] Figure 20 A schematic diagram illustrating the principle of testing a toroidal suspended MEMS device for stable and transient thermal conductivity. Detailed Implementation

[0029] like Figure 1 As shown, the test structure in this embodiment includes a silicon substrate 14 and a silicon nitride support film 1. The silicon substrate has a hollow structure in the middle, with a second annular suspended platform structure 7 and a first annular suspended platform structure 8 serving as electrical and thermal insulating substrates for the thermocouple layers. Above the two annular suspended platforms are a second thermocouple stack temperature sensor, a first thermocouple stack temperature sensor, a second metal heating electrode 3, a first metal heating electrode 9, a second pyroelectric thin film 7, and a first pyroelectric thin film 13. The structures on the left and right sides are completely symmetrical. Each pair of thermocouples is composed of p-type polycrystalline / n-type polycrystalline silicon materials, and is formed by three-dimensionally stacking upper thermocouple parts (n-type polycrystalline silicon) 6, 12 and lower thermocouple parts (p-type polycrystalline silicon) 5, 11 in a spatial scale. Silicon nitride is used in the middle to provide electrical isolation between the two thermocouple layers, preventing short circuits between the upper and lower thermocouples. An rGO redox graphene thermal rectifier layer is designed in the middle of the silicon nitride to increase the temperature difference between the two ends of the thermocouple and improve measurement accuracy. Each thermopile has a pair of pads 4 and 10 for measuring the thermopile's voltage output. The lower surface of the pyroelectric thin film is connected to a p-type polysilicon strip 19 and 20, and the upper surface is connected to an n-type polysilicon strip 16 and 17. Pads are extended outside the suspended platform for measuring the pyroelectric current. After the thermopile and pyroelectric structure are completed, a silicon nitride thin film is deposited as a protective layer. Finally, a metal heating electrode is deposited on the silicon nitride-encased suspended platform as a heat source.

[0030] During testing, the sample material is transferred between the second annular suspended platform 7 and the first annular suspended platform 8, with both ends fixed at arbitrary positions on the two platforms. Thermopile temperature sensors and pyroelectric transient temperature sensors are located nearby to make the sample temperature closer to the test temperature, ensuring the accuracy of the measurement.

[0031] In this embodiment, the two annular suspended platforms of the steady-state and transient thermal conductivity testing annular suspended MEMS device are in a vacuum environment, so the heat conduction in the vacuum can be ignored, and only the cantilever conducts heat to the environment. This can maintain the hot and cold junction while effectively improving the sensitivity of the sensor.

[0032] The specific manufacturing method of this embodiment is as follows:

[0033] Step 1, as follows Figure 3 As shown, a silicon nitride layer is prepared on both sides of the silicon substrate 14 by chemical vapor deposition. The upper silicon nitride layer 1 serves as the substrate for the electrical and thermal insulating layer and the metal layer, and the lower silicon nitride layer 15 serves as the mask for etching the silicon substrate.

[0034] Step 2, as follows Figure 4 As shown, the right rGO reduction graphene layer 25 and the left rGO reduction graphene layer 26 were prepared by chemical vapor deposition combined with the Hummers method as thermal rectification layers.

[0035] Step 3, as follows Figure 5 As shown, a layer of silicon nitride 27 was prepared by chemical vapor deposition as an electrical insulating layer between graphene and thermocouple layer.

[0036] Step 4, as follows Figure 6 As shown, a first polysilicon layer (p-type polysilicon) is prepared by plasma etching to form the lower thermocouple portion 5 of the second five-pair thermocouple temperature sensor and the lower thermocouple portion 19 of the second pyroelectric temperature sensor, as well as the lower thermocouple portion 11 of the first five-pair thermocouple temperature sensor and the lower thermocouple portion 20 of the first pyroelectric temperature sensor.

[0037] Step 5, as follows Figure 7 As shown, the second pyroelectric 7 and the first pyroelectric 13 are prepared by plasma etching process. The lower surface of the pyroelectric film (barium titanate or other pyroelectric material) is in contact with a lower thermocouple for pyroelectric current extraction.

[0038] Step 6, as follows Figure 8 As shown, a layer of silicon nitride 21 is prepared by chemical vapor deposition as an electrical insulation layer for the upper and lower thermocouples, while protecting the lower polysilicon layer from being etched by the upper polysilicon layer etching process.

[0039] Step 7, as follows Figure 9 As shown, a second polysilicon layer (n-type polysilicon) is prepared by plasma etching to form the upper thermocouple portion 6 of the second five-pair thermocouple temperature sensor and the upper thermocouple portion 17 of the second pyroelectric temperature sensor, as well as the upper thermocouple portion 12 of the first five-pair thermocouple temperature sensor and the upper thermocouple portion 16 of the first pyroelectric temperature sensor.

[0040] Step 8, as follows Figure 10 As shown, the silicon nitride layer is etched to form an opening, exposing the hot and cold junctions of the lower thermocouple of the thermocouple temperature sensor, which facilitates the subsequent metal electrical connection.

[0041] Step 9, as follows Figure 11As shown, plasma etching is used to pattern the Pt metal. The first and second sides are used to connect the hot junction metal portions 22 and 23 of the thermocouple temperature sensor, which require connecting the upper thermocouple portions 6 and 12 to the lower thermocouple portions 5 and 11. The first and second sides are used to connect the cold junction metal portions 24, 4, 25, and 21 of the thermocouple temperature sensor, which require connecting the upper thermocouple metal portions 24 and 25 of this thermocouple pair to the lower thermocouple metal portions 4 and 10 of the adjacent thermocouple pair, to achieve series connection of the thermocouples and increase the output electromotive force.

[0042] Step 10, as follows Figure 12 As shown, a layer of silicon nitride 26 is prepared by chemical vapor deposition to protect the first and second polysilicon layers from being damaged in subsequent silicon substrate etching processes.

[0043] Step 11, as follows Figure 13 As shown, a second serpentine metal heating electrode 3 and a first serpentine metal heating electrode 9 for heating a thermocouple temperature sensor are fabricated using a plasma etching process.

[0044] Step 12, as follows Figure 14 As shown, the silicon nitride layer is etched to form openings, exposing the metal pads 4 and 10. The middle portion of the silicon nitride is completely etched to achieve isolation between the two islands.

[0045] Step 13, as follows Figure 15 As shown, silicon nitride 15 is etched on the back side to form an etched window.

[0046] Step 14, as follows Figure 16 As shown, a hollow structure is formed by etching the silicon substrate 14 from the bottom, releasing the annular suspended platform and the fan-shaped thermocouple cantilever.

[0047] Figure 17 and Figure 18 The simulation results show the temperature of a traditional MEMS resistance temperature detector (RTD) thermal bridge structure and a toroidal suspended MEMS device for steady-state and transient thermal conductivity testing. The heating end temperature of both devices is 300 K, the sensor end temperature is 294 K, and the ambient temperature is 290 K. The simulation results show that although a temperature gradient exists within the toroidal suspended platform when using the steady-state and transient thermal conductivity testing toroidal suspended MEMS device, the smaller size of the toroidal suspended platform allows the temperature detection point to be closer to both ends of the sample compared to the traditional MEMS RTD thermal bridge structure. Figure 15Temperature simulations of a traditional MEMS resistance-to-thermal bridge structure show the following: the heating platform has an average temperature of 297.68 K and a temperature difference of 2.32 K; the sensor platform has an average temperature of 293.07 K and a temperature difference of 0.93 K; the heating resistor has an average temperature of 296.2 K with a relative error of 1.267%; and the sensor has an average temperature of 292.48 K with a relative error of 0.517%. Figure 16 Temperature simulations of the toroidal suspended MEMS device, used for steady-state and transient thermal conductivity testing, show that the average temperature of the heating platform is 299.31 K with a temperature difference of 0.69 K, and the average temperature of the sensor platform is 293.72 K with a temperature difference of 0.28 K.

[0048] Figure 19 Simulation diagram of the potential of the temperature sensor when measuring temperature in a toroidal suspended MEMS device for stable and transient thermal conductivity testing. Figure 16 The average temperature measured at the hot end of the thermopile on the heating platform can be calculated to be 298.21 K. Figure 17 From the potential diagram and the Seebeck effect, we can calculate:

[0049] V h1 = 1.2368mV, V h2 = -10.803mV.

[0050]

[0051] The relative error is only 0.148%.

[0052] Depend on Figure 18 The average temperature measured at the hot end of the thermopile on the sensor platform can be calculated to be 293.32 K. Figure 19 From the potential diagram and the Seebeck effect, we can calculate:

[0053] V s1 =3.6371mV, V s2 = -1.1666mV.

[0054]

[0055] The relative error is only 0.135%.

[0056] The simulation results show that the temperature gradient measured by the toroidal suspended MEMS device for steady-state and transient thermal conductivity testing is smaller than that of the traditional resistance temperature detector (RTD) suspended thermal bridge device, and is closer to the actual measured temperature. The accuracy is also significantly improved compared to the thermal bridge method. At the same time, the output electromotive force is in the millivolt range, which is easier to measure. Therefore, it helps to improve the accuracy of thermal conductivity measurement.

[0057] The method for measuring the thermal conductivity of a sample using a toroidal suspended MEMS device with steady-state and transient thermal conductivity testing is as follows:

[0058] Figure 20 This is a schematic diagram illustrating the principle of thermal conductivity measurement using a toroidal suspended MEMS device with steady-state and transient thermal conductivity testing capabilities. The entire suspended device is placed in a vacuum system, thus the heat generated by thermal convection and thermal radiation can be considered negligible.

[0059] (1) Steady-state thermal conductivity test:

[0060] First, the sample is placed between the second annular suspended platform and the first annular suspended platform, assuming an initial temperature of T0. The second metal heating electrode 3 is heated using direct current, causing the temperature of the second annular suspended platform to rise to T0. h The Joule heat generated in this process is Q. h The direct current also heats the two supporting silicon nitride cantilever arms, generating Joule heat of 2Q1. Half of this heat flows along the cantilever arms to the external environment, and the other half flows to the second annular suspended platform 2. Therefore, the total heat generated by the direct current is Q. h +2Q1. According to the law of conservation of energy, the heat generated is equal to the heat dissipated. Heat dissipation occurs through two pathways: some heat is conducted through the sample to the first annular suspended platform, causing the temperature of the first annular suspended platform to rise to T. s The heat is then transferred to the external environment T0 through the seven support arms of the first annular suspended platform; let this portion of heat be heat Q2. Another portion is transferred to the external environment T0 through the seven support arms of the second suspended platform; this portion of heat is Q. h +2Q1-Q2. Assuming the thermal conductivity of a single cantilever is Gb, it can be calculated from the cantilever thermal conductivity K. b The cross-sectional area S and cantilever length L are calculated as follows:

[0061] G b =K b S / L

[0062] Heat can be represented by the product of the material's thermal conductivity and the temperature difference; therefore, the heat flowing from a single cantilever to the external environment can be expressed as:

[0063] Q1 = Gb × (Th – T0)

[0064] Since the heat conducted through the sample is the same as the heat transferred to the external environment by the first annular suspended platform through the cantilever, therefore:

[0065] Q2=Gs×(Th–Ts)=5Gb×(Ts–T0)

[0066] The heat transferred to the environment by the two support arms that apply DC power through the second annular suspended platform is:

[0067]

[0068] The heat transferred to the environment through the five support arms of the second annular suspended platform without the application of DC power is:

[0069] Q s2 =5Q1=5[Gb×(Th–T0)]

[0070] From the conservation of total heat, we get:

[0071] Q h +2Q1-Q2=Q s2 +Q s2

[0072] Substituting into the above equations, we get:

[0073]

[0074] Similarly, the thermal conductivity of the sample can also be calculated:

[0075]

[0076] Where L is the length of the sample, w is the width of the sample, and t is the thickness of the sample. The temperature T of the second suspended platform... h The temperature T of the first suspended heat island 104 can be measured by any combination of the second and fifth pairs of temperature sensors. s The temperature can be measured by any combination of the second and fifth pairs of temperature sensors. For example, the temperature corresponding to each pair of thermocouple temperature sensors can be calculated from Table 1.

[0077] Table 1. Description of Thermocouple Temperature Sensor Pad Connections and Temperature Measurement Points

[0078]

[0079]

[0080] (2) Transient thermal conductivity test:

[0081]

[0082] The generated pyroelectric current, Q is the induced charge, A is the electrode surface area of ​​the material, and p is the pyroelectric coefficient.

[0083] Differentiating the thermal conductivity, we get:

[0084]

[0085] Substituting into the pyroelectric current formula, we get:

[0086]

[0087] Among them, the pyroelectric current I psI ph It can be measured by an external voltmeter and resistor.

Claims

1. A ring-shaped suspended MEMS device for in-situ thermal conductivity testing, characterized in that, Includes a silicon substrate and a silicon nitride support film; The silicon substrate has a hollow structure in the middle, and a first annular suspended platform structure and a second annular suspended platform structure are provided on the upper surface of the silicon substrate as an electrically and thermally insulating substrate for the thermocouple layer. The first annular suspended platform structure and the second annular suspended platform are provided with a first thermocouple stack temperature sensor, a second thermocouple stack temperature sensor, a first metal heating electrode, a second metal heating electrode, a first pyroelectric film, a second pyroelectric film and thermocouple pairs on the cantilever on the upper surface of the platform. The metal heating electrode and the pyroelectric thin film are connected to the electrical signal of the external electrode and then embedded in a thermal isolation island made of ring-shaped silicon nitride. A silicon nitride fan-shaped rGO redox graphene interlayer is designed between the annular suspended platform and the heat sink connected to the electrical signal to serve as a thermal rectifier layer to improve the temperature difference between the two ends of the thermocouple. During testing, the sample material within a certain length range is transferred and mounted between the second annular suspended platform and the first annular suspended platform, with both ends fixed at any position on the second annular suspended platform and at any position on the first annular suspended platform, respectively. Any pair of thermocouples can output.

2. The annular suspended MEMS device for in-situ thermal conductivity testing according to claim 1, characterized in that: The second annular suspended platform and the first annular suspended platform are identical in volume and structure. They are arranged in a centrally symmetrical ring and supported by 7 pairs of cantilever arms, including 5 pairs of fan-shaped cantilever arms and 2 pairs of straight cantilever arms, all of which are made of silicon nitride.

3. The annular suspended MEMS device for in-situ thermal conductivity testing according to claim 1, characterized in that: The five pairs of sector-shaped cantilever arms contain rGO redox graphene interlayers. By utilizing the thermal rectification principle caused by size asymmetry, the direction of heat flow is controlled. Heat transfer from the hot end of the thermopile in the annular suspended platform to the cold end of the heat sink is more difficult and concentrated at the hot end, thus improving measurement accuracy.

4. The annular suspended MEMS device for in-situ thermal conductivity testing according to claim 1, characterized in that: The metal layer consists of an upper metal layer and a lower metal layer; The upper metal layer consists of a second metal heating electrode and a first metal heating electrode, which are located above the second annular suspended platform and the first annular suspended platform, respectively. The lower metal layer is a metal connection layer, which is in direct contact with the hot and cold junctions of the thermocouple and is used to connect the hot junction of the thermocouple.

5. The annular suspended MEMS device for in-situ thermal conductivity testing according to claim 1, characterized in that: The thermocouple stack temperature sensor consists of five pairs of thermocouple polycrystalline silicon strips. Each pair of thermocouples is formed by stacking p-type polycrystalline silicon and n-type polycrystalline silicon strips on top of each other, with a silicon nitride thin film in between as an isolation. The whole structure is located on a silicon nitride support substrate. The two types of polycrystalline silicon strips are joined together with metal at the hot end to form a thermocouple junction; the cold ends are joined together with metal to form a thermocouple stack in series.

6. The annular suspended MEMS device for in-situ thermal conductivity testing according to claim 1, characterized in that: n-type polycrystalline silicon is the upper thermocouple material, and p-type polycrystalline silicon is the lower thermocouple material. The difference in their Seebeck coefficients is 3.1 × 10⁻⁶. -4 V / K.

7. The annular suspended MEMS device for in-situ thermal conductivity testing and its fabrication method according to claim 1, characterized in that: The pyroelectric thin film is made of barium titanate or other pyroelectric materials. When the temperature of the annular suspended platform changes, the polarization intensity inside the self-polarized crystal material decreases, and a current is generated on the surface of the material. The pyroelectric current can be measured by connecting it to the outside. It is directly proportional to the change of temperature over time and is used to measure transient thermal conductivity.

8. The method for fabricating a toroidal suspended MEMS device for in-situ thermal conductivity testing according to claim 1, characterized in that, The preparation method includes the following steps: Step 1: A silicon nitride layer is prepared on both sides of the silicon substrate by chemical vapor deposition. The upper silicon nitride layer serves as the substrate for the electrical and thermal insulating layer and the metal layer, and the lower silicon nitride layer serves as the mask for etching the silicon substrate. Step 2: The first polysilicon layer is prepared by plasma etching process. The first polysilicon layer is p-type polysilicon. Step 3: The first pyroelectric layer and the second pyroelectric layer are prepared by plasma etching process. The second pyroelectric layer is barium titanate or other pyroelectric materials. Step 4: Prepare a layer of silicon nitride by chemical vapor deposition as an electrical insulation layer for the upper and lower thermocouples, while protecting the lower polysilicon layer from being etched by the upper polysilicon layer etching process. Step 5: A second polysilicon layer is prepared using a plasma etching process. The second polysilicon layer is an n-type polysilicon. Step 6: Etch the silicon nitride layer to form openings for electrical connections to the metal layer; Step 7: Use plasma etching process to pattern the metal Pt electrode; Step 8: Prepare a layer of silicon nitride by chemical vapor deposition to protect the first and second polysilicon layers from being damaged in the subsequent silicon substrate etching process; Step 9: Fabricate a Pt metal heating electrode using plasma etching. Step 10: Etch the silicon nitride layer to form openings and expose the metal pads; Step 11: Etch silicon nitride on the back side to form an etched window; Step 12: Etch the silicon substrate from the bottom to form a hollow structure, releasing the annular suspended platform and thermocouple cantilever.

9. The preparation method according to claim 8, characterized in that the device is used for in-situ thermal conductivity testing using a scanning electron microscope, and simultaneously realizes the microstructural relationship between thermal conductivity and sample size and doping.

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