A MEMS gyroscope based on bi-directional thermal expansion flow

Abstract

This invention relates to a MEMS gyroscope based on bidirectional thermal expansion flow, belonging to the field of inertial measurement. The gyroscope mainly comprises a base layer, a sensitive layer, and a cover plate. Three sets of parallel heaters and two pairs of parallel thermistors are disposed on the upper surface of the sensitive layer. A suspended insulating resistance bridge is disposed below the heaters and thermistors. An "I"-shaped cavity is etched on the upper surface of the base layer. Grooves are etched on the cover plate, which is sealed to the upper surface of the sensitive layer. This invention is fabricated using MEMS processing technology, offering advantages such as small size, low cost, and mass production capability. Based on these advantages, it can be widely applied in electronic devices, aerospace, and medical instruments.

Description

Technical Field

[0001] This invention relates to the technical field of detecting the angular velocity and attitude parameters of a moving body using a Coriolis force deflection heat flow sensor, and particularly to a MEMS gyroscope based on bidirectional thermal expansion flow, belonging to the field of inertial measurement. Background Technology

[0002] A gyroscope is an inertial sensor used to accurately measure the angular velocity of a moving object. Silicon-based microstructure gyroscopes are among the most common sensors, including pressure sensors and accelerometers. Microstructure gyroscopes have been extensively researched and developed over the past few decades, and with the advent of microelectromechanical systems (MEMS) technology, these sensors have gradually matured. Traditional micro-gyroscopes (micromechanical gyroscopes) are miniaturized rate gyroscopes combining microelectronics and micromechanics, based on the Coriolis effect principle where a high-frequency vibrating mass is rotated by a base. The solid mass within the sensing element of such gyroscopes needs to be suspended and vibrated by a mechanical elastic body, making them susceptible to damage under slightly high acceleration impacts. Furthermore, vacuum encapsulation is required to reduce damping, resulting in complex manufacturing processes and fatigue damage and vibration noise over prolonged operation. In contrast, microfluidic inertial devices are a new type of device that measures input acceleration and angular velocity by detecting the flow field shift of fluid within a sealed cavity. Because they lack the moving parts and suspension system found in traditional micro-gyroscopes, they can withstand high overloads; because their sensing mass is gas, with almost zero mass, they have short response times and long lifespans; and due to their simple structure, they meet the requirements of low-cost applications.

[0003] The sensing element of a miniature inertial sensor based on the principle of thermal expansion is a gas. It obtains the external angular velocity by sensing the temperature difference of the fluid subjected to angular velocity through a temperature sensor. Because it eliminates the suspension mass and vibration structure of traditional accelerometers, it can withstand high impacts while maintaining a certain level of accuracy, effectively resolving the contradiction between high overload and high precision. Furthermore, due to its fabrication using MEMS technology, it boasts advantages such as small size, light weight, and low cost, enabling the widespread application of thermal expansion miniature inertial sensors. The principle of MEMS thermal expansion gyroscopes is a pioneering innovation internationally. Its simple structure and manufacturing process, extremely low cost, high reliability, and excellent vibration and shock resistance make it possible for it to compete with capacitive micromechanical vibratory gyroscopes in the low-precision, low-price miniature gyroscope market. Based on these advantages, it can be widely used in platform stabilization systems, such as those for cameras and camcorders, resulting in a very bright market prospect. More importantly, it can be combined with thermal convection accelerometers to form applications such as inertial guidance systems resistant to high impacts, and its range and sensitivity are not limited by traditional theories.

[0004] The working principle of a miniature thermal expansion gyroscope is based on the flow velocity of a flow field to measure angular velocity. When the heater is heated by a driving voltage, the gas above the heater rises due to the heat, causing airflow from both sides to replenish it, resulting in a flow close to the thermistor. When there is no external angular velocity, the gas flow velocities on both sides of the thermistor are equal and opposite in direction, resulting in a perfectly symmetrical flow field distribution. The temperature sensors detect the same temperature, and the detection circuit outputs a zero angular velocity. When an angular velocity signal in the Z direction is applied, a Coriolis acceleration in the Y direction is generated on the gas moving in the X direction. This acceleration causes the gas motion to deviate in the Y direction, resulting in different changes in the temperature sensors at symmetrical positions in the Y direction. This change is transmitted through a Wheatstone bridge, outputting a voltage proportional to the input angular velocity, thus obtaining the angular velocity value. Chinese patent application No. 202010584315.0 describes a single-heat-source convection micromechanical Z-axis thin-film gyroscope. This gyroscope utilizes a set of heaters that alternately heat the surface, creating an asymmetric temperature field due to Coriolis force. A symmetrical thermistor is used to detect this asymmetric temperature distribution. However, due to the arrangement of the heaters and thermistors in these thermal expansion gyroscopes, some of the heat generated during heating is lost, resulting in a small temperature difference and low sensitivity. Furthermore, most current thermal expansion gyroscopes use either a pair of heaters for cross-heating or a single heat source, which can lead to coupling effects between linear acceleration and rotational angular velocity, as well as asymmetric forces on the fluid within the cavity, hindering rapid flow. These issues affect sensor accuracy and limit its application. Therefore, overcoming these problems is a crucial technical challenge that needs to be addressed by those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide a MEMS gyroscope based on bidirectional thermal expansion flow to solve the technical problems existing in the prior art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention provides a MEMS gyroscope based on bidirectional thermal expansion flow, comprising a base layer, a sensitive layer, and a cover plate, wherein,

[0008] An "I"-shaped cavity is etched on the upper surface of the base layer; a groove is etched on the cover plate and is sealed to the upper surface of the sensitive layer; the base layer and the cover plate isolate the working gas inside the "I"-shaped cavity from the outside, forming a working space for the sensitive gas; the height of the "I"-shaped cavity and the depth of the groove on the cover plate are the total cavity height H, 50μm≤H≤300μm;

[0009] The upper surface of the sensitive layer is provided with three pairs of heaters and two pairs of parallel thermistors, and a suspended insulating resistance bridge is provided below the heaters and thermistors.

[0010] The direction parallel to the thermistor on the upper surface of the sensitive layer is defined as the X direction, the direction perpendicular to the thermistor is defined as the Y direction, and the height direction of the sensitive layer is defined as the Z direction.

[0011] The heater and the thermistor are both placed perpendicular to the Y direction; four thermistors are used to detect the Z-axis angular velocity and are placed along the X-axis, with their placement direction perpendicular to the Y-axis; three pairs of heaters are symmetrically placed along the X-axis, with their placement direction perpendicular to the Y-axis.

[0012] The three pairs of heaters are energized in a periodic push-pull manner, meaning that one working cycle of the heater includes the pulse voltage excitation time and the power-off interval time.

[0013] As a further technical solution, each of the heaters is driven by two square wave signals of the same frequency, with a phase difference of 90 degrees and a pulse duty cycle of 50%.

[0014] As a further technical solution, the outer edge of the "I"-shaped cavity is larger than the outer contour of the upper surface heater and thermistor.

[0015] As a further technical solution, the height of the "I"-shaped cavity is 2 / 3 to 3 / 4 of the height of the entire sensitive layer.

[0016] As a further technical solution, the depth of the groove etched on the cover plate is 2 / 3 of the total height of the cover plate.

[0017] As a further technical solution, the height of the heater and thermistor on the upper surface of the sensitive layer is h, where 0.5μm≤h≤2μm.

[0018] As a further technical solution, each heater is composed of 6 Cr / Pt metal resistance wires connected in series.

[0019] As a further technical solution, each of the thermistors is composed of four Cr / Pt metal resistance wires connected in series.

[0020] As a further technical solution, the MEMS gyroscope based on bidirectional thermal expansion flow of the present invention can be manufactured using MEMS technology, and its specific processing method is as follows:

[0021] Step 1: Thermally oxidize a 500nm SiO2 thin film on both sides of the silicon wafer;

[0022] Step 2: Spin-coat positive photoresist onto both sides of the substrate;

[0023] Step 3: A single photolithography step is used to create the groove-etched window;

[0024] Step 4: Wet etching of SiO2 below the window;

[0025] Step 5: Remove the photoresist using NMP solution (N-methylpyrrolidone);

[0026] Step 6: Wet etching of Si to form a groove structure;

[0027] Step 7: Apply a second coat of adhesive to the front side of the substrate;

[0028] Step 8: Secondary photolithography to form the heater, thermistor and pads;

[0029] Step 9: Sputter Cr / Pt / Au as metal layers respectively;

[0030] Step 10: Peel off to form the desired structure of the thermal element.

[0031] By adopting the above technical solution, the present invention has the following beneficial effects:

[0032] 1. The present invention proposes a MEMS gyroscope based on bidirectional thermal expansion flow, which inherits the advantages of traditional gyroscopes such as strong shock resistance, simple structure, low cost and low power consumption, and realizes the Z-axis angular velocity measurement of MEMS gyroscope based on thermal expansion flow.

[0033] 2. This MEMS gyroscope based on bidirectional thermal expansion flow consists of three alternately heated heaters and four thermistors. Due to its unique structural advantages and asynchronous driving mode, the fluid formed in it can move faster in both directions and symmetrically in the chamber. Therefore, it can achieve higher output sensitivity and better suppression of cross coupling.

[0034] 3. The process used in this invention is compatible with integrated circuit technology, is simple, has a high yield of sensitive components, and is easy to introduce into microcomputer embedded systems (microcontrollers) for temperature compensation and nonlinearity compensation, and has the potential for high integration. Attached Figure Description

[0035] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0036] Figure 1A schematic diagram of a three-dimensional MEMS gyroscope based on bidirectional thermal expansion flow is provided for an embodiment of the present invention.

[0037] Figure 2 A schematic diagram of the three-dimensional structure of the base layer provided in an embodiment of the present invention;

[0038] Figure 3 This is a schematic diagram of the three-dimensional structure of the cover plate provided in an embodiment of the present invention;

[0039] Figure 4 This is a top view of the base layer and sensitive layer provided in an embodiment of the present invention;

[0040] Figure 5 for Figure 4 Sectional view along axis AA;

[0041] Figure 6 A schematic diagram illustrating the working principle of an embodiment of the present invention;

[0042] Figure 7 This is a schematic diagram of the heater provided in an embodiment of the present invention;

[0043] Figure 8 This is a schematic diagram of the structure of a thermistor provided in an embodiment of the present invention;

[0044] Figure 9 This invention provides a flowchart of a novel micromechanical Z-axis thermal expansion gyroscope fabrication process.

[0045] Icons: 1-Base layer, 2-Sensitive layer, 3-I-shaped cavity, 4-Insulation resistance bridge, 5-Cover plate, 6-Cover plate groove, 7-Heater, 8-Heater, 9-Heater, 10-Heater, 11-Heater, 12-Heater, 13-Thermistor, 14-Thermistor, 15-Thermistor, 16-Thermistor, 17-Metal electrode, 18-Heating resistance wire, 19-Thermistor wire. Detailed Implementation

[0046] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0048] Combination Figure 1-6As shown in the figure, this embodiment of the present invention provides a MEMS gyroscope based on bidirectional thermal expansion flow, including a base layer 1, a sensitive layer 2 and a cover plate 5, wherein an "I"-shaped cavity 3 is etched on the upper surface of the base layer 1; a groove 6 is etched on the cover plate 5 and is sealed to the upper surface of the sensitive layer 2; the base layer 1 and the cover plate 5 isolate the working gas in the "I"-shaped cavity 3 from the outside world, forming a working space for the sensitive gas.

[0049] The upper surface of the base layer 1 is provided with three pairs of heaters (heater 7 and heater 8, heater 9 and heater 10, heater 11 and heater 12) and two pairs of thermistors (thermistors 13 and thermistors 14, thermistors 15 and thermistors 16) arranged in parallel with each other, and a suspended insulating resistance bridge 4 is provided below the heaters and thermistors.

[0050] The direction parallel to the thermistor on the upper surface of the sensitive layer is defined as the X direction, the direction perpendicular to the thermistor is defined as the Y direction, and the height direction of the sensitive layer is defined as the Z direction; the heater and the thermistor are both placed in an orientation parallel to the X direction.

[0051] Four thermistors are used to detect the Z-axis angular velocity: thermistor 13, thermistor 14, thermistor 15, and thermistor 16. They are placed along the X-axis direction and their placement direction is perpendicular to the Y-axis direction.

[0052] Three pairs of heaters (heater 7 and heater 8, heater 9 and heater 10, heater 11 and heater 12) are placed parallel to each other along the Y-axis, with their placement direction parallel to the X-axis.

[0053] During operation, three pairs of resistance heaters heat the gas medium and promote the directional movement of the gas flow along the X-axis. The heaters are driven by two square waves of the same frequency with a 90-degree phase difference and a pulse duty cycle of 50%. The thermistors are energized with a constant current.

[0054] Specifically: Inside the sealed cavity, the heater resistor generates Joule heat, releasing heat to the surrounding gas, causing thermal diffusion and forming a moving thermal expansion flow; while the square wave acting on the heater alternately heats and cools the three sets of heaters, thus forming a symmetrical, push-pull, bidirectional thermal expansion flow between the three sets of heaters (e.g., Figure 6 As shown in the diagram, this symmetrical thermal expansion flow exhibits high velocity and stable airflow, effectively suppressing cross-coupling with acceleration. Four thermistors arranged in parallel detect changes in the surrounding gas temperature caused by external angular velocity input, while simultaneously increasing the contact area of ​​the thermal expansion flow, thus enhancing the gyroscope's sensitivity.

[0055] Specifically, when there is an external Z-axis angular velocity input, due to the Coriolis force principle, the thermal expansion flow of the motion is deflected accordingly. The hot air flow generated by the three sets of heaters in the Y-axis direction will reach the corresponding two sets of parallel thermistors in opposite directions, forming opposite heating effects. The thermistors generate a temperature difference proportional to the input Z-axis angular velocity. According to the metal resistance effect, the thermistors will generate a resistance difference. Through the Wheatstone bridge circuit, the detected resistance difference is converted into a voltage difference. Then, the magnitude of the external Z-axis angular velocity can be calculated from the temperature difference and voltage difference.

[0056] In this embodiment, as a further technical solution, each heater is driven by two square wave signals of the same frequency with a phase difference of 90 degrees and a pulse duty cycle of 50%. The resistance generates Joule heating, releasing heat to the surrounding gas, causing thermal diffusion and forming a heat flow. This heat flow acts on the square wave on the heater, alternately heating and cooling each pair of heaters. Thus, between the three sets of heaters, a push-pull bidirectional thermal expansion flow is formed between every two sets, characterized by high flow velocity and stable airflow. The square wave-driven heater operation is divided into two stages. In the first stage, heaters 7, 8, 11, and 12 are energized and heating, while heaters 9 and 10 are de-energized and at ambient temperature, generating a thermal expansion flow pointing towards heaters 9 and 10. In the second stage, heaters 9 and 10 are energized and heating, while heaters 7, 8, 11, and 12 are de-energized and at ambient temperature, generating a thermal expansion flow in the opposite direction to that in the first stage, pointing towards heaters 7, 8, 11, and 12.

[0057] The working principle of a single-axis thermal flux gyroscope is explained using the first stage as an example. When there is an angular velocity input Ωz in the Z-axis direction, due to the Coriolis force, the two sets of thermal expansion flows generated between heater 7 and heater 8, heater 9 and heater 10, and heater 11 and heater 12 will be deflected in the YOX plane. The temperature of the thermistor deflected by the thermal expansion flow is higher than that of the other thermistor collinear with it. Therefore, the two sets of thermistors 13, 14 and 15, 16, arranged in a straight line, generate a temperature difference proportional to the input angular velocity Ωz. Thermistors 13, 14 and 15, 16 are connected to form two equal arms of two Wheatstone bridges. Heating will change the resistance of the thermistor lines. The change in resistance is converted into two voltage Vz outputs proportional to the angular velocity Ωz through the Wheatstone bridge, thus sensing the Z-axis angular velocity.

[0058] In this embodiment, as a further technical solution, the outer edge of the "I"-shaped cavity is larger than the outer contour of the upper surface heater and thermistor to form a thin film structure, thereby increasing the thermal diffusion of the gas medium in the sealed cavity.

[0059] In this embodiment, as a further technical solution, the height of the "I"-shaped cavity is 2 / 3 to 3 / 4 of the height of the entire sensitive layer.

[0060] In this embodiment, as a further technical solution, the depth of the groove etched on the cover plate is 2 / 3 of the total height of the cover plate.

[0061] In this embodiment, as a further technical solution, the height of the heater and thermistor on the upper surface of the sensitive layer is h, where 0.5μm≤h≤2μm.

[0062] Combination Figure 7-8 As shown, in this embodiment, as a further technical solution, each heater is composed of six Cr / Pt material heating resistance wires 18 connected in series. This design of the Cr / Pt material heater resistance wires allows the heater to generate more heat, thereby improving the sensitivity of the gyroscope detection. Each thermistor is composed of four Cr / Pt material thermistor wires 19 connected in series. This design of the Cr / Pt material thermistor resistance wires allows the thermistor to obtain a larger voltage signal output, thereby improving the sensitivity of the gyroscope detection. Each electrode in this application is composed of Cr / Au material metal resistors 17; this design of the metal electrodes increases the stability and shock resistance of the gyroscope.

[0063] In summary, the thermal expansion gyroscope proposed in this invention employs alternating heating and cooling with three sets of heaters and uses an asynchronous drive method, enabling the fluid formed to move faster and symmetrically in two directions within the chamber. It offers advantages such as higher sensitivity, stronger suppression of cross-coupling, simple structure, small size, and high reliability. Furthermore, the "I"-shaped cavity 3 results in a very thin base layer, providing excellent heat dissipation. The process used in this invention is compatible with integrated circuit technology, exhibits good stability, and can measure planar Z-axis angular velocity. It possesses high integration and mass production potential, and is characterized by small size, low power consumption, and low cost.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A MEMS gyroscope based on bidirectional thermal expansion flow, characterized in that, It includes a base layer, a sensitive layer, and a cover plate, among which, An "I"-shaped cavity is etched on the upper surface of the base layer; a groove is etched on the cover plate and is sealed to the upper surface of the sensitive layer; the base layer and the cover plate isolate the working gas in the "I"-shaped cavity from the outside, forming a working space for the sensitive gas; the height of the "I"-shaped cavity and the depth of the groove on the cover plate are the total cavity height H, 50μm≤H≤300μm; The upper surface of the substrate layer is provided with three pairs of heaters that are parallel to each other and two pairs of thermistors that are arranged in parallel. A suspended insulating resistance bridge is provided below the heaters and the thermistors. The three pairs of heaters use alternating heating and cooling and are driven asynchronously. Define the direction of the upper surface of the sensitive layer that is parallel to the thermistor as the X direction, the direction that is perpendicular to the thermistor as the Y direction, and the height direction of the sensitive layer as the Z direction. The heaters and thermistors are placed perpendicular to the Y-direction; four thermistors are used to detect the Z-axis angular velocity, placed along the X-axis, with their placement direction perpendicular to the Y-axis; three pairs of heaters are symmetrically placed along the X-axis, with their placement direction perpendicular to the Y-axis. The three pairs of heaters are energized in a periodic push-pull manner, meaning that one working cycle of the heater includes the pulse voltage excitation time and the power-off interval time. Each pair of heaters is driven by two square wave signals of the same frequency, with a phase difference of 90 degrees and a pulse duty cycle of 50%. The outer edge of the "I"-shaped cavity is larger than the outer contour of the upper surface heater and thermistor.

2. A MEMS gyroscope based on bi-directional thermal expansion flow according to claim 1, characterized in that, The height of the "I"-shaped cavity is 2 / 3 to 3 / 4 of the total height of the sensitive layer.

3. A MEMS gyroscope based on bidirectional thermal expansion flow according to claim 1, characterized in that, The depth of the groove etched on the cover plate is 2 / 3 of the total height of the cover plate.

4. A MEMS gyroscope based on bi-directional thermal expansion flow according to claim 1, wherein, The height of the heater and thermistor on the upper surface of the sensitive layer is h, where 0.5μm≤h≤2μm.

5. A MEMS gyroscope based on bi-directional thermal expansion flow according to claim 1, wherein, Each heater consists of six Cr / Pt metal resistance wires connected in series.

6. A MEMS gyroscope based on bi-directional thermal expansion flow according to claim 1, wherein, Each of the thermistors consists of four Cr / Pt metal resistance wires connected in series.

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