A thermal-capacitive wide-range MEMS wind speed and direction sensor

Abstract

This invention discloses a thermal-capacitive wide-range MEMS wind speed and direction sensor. The sensor chip consists of a thermal resistor and a capacitor, connected to a Wheatstone bridge to convert wind speed signals into electrical signals for measurement. The thermal resistor employs a self-heating mode, providing a symmetrical thermal field while simultaneously measuring temperature. Under wind conditions, the symmetrical thermal field on the chip surface is disrupted, and measuring the change in resistance value allows for the measurement of wind direction at low speeds. At high wind speeds, the thermal principle fails to measure due to saturation. The capacitor, composed of a cantilever beam, retains a certain degree of warping after release. Under wind conditions, the warping angle changes, and the change in capacitance value characterizes the deformation of the cantilever beam caused by the wind, thereby obtaining high-speed wind direction information. This invention provides a new solution to the problem of thermal sensors being unable to measure high-speed wind direction without increasing chip area, ensuring measurement accuracy for both high and low wind speeds within the specified range.

Description

Technical Field

[0001] This invention discloses a wide-range MEMS wind speed and direction sensor based on thermal and capacitive methods, which is designed to measure wind speed at both low and high speeds and belongs to the technical field of measurement and testing. Background Technology

[0002] Wind speed and direction are crucial parameters, and accurate and rapid measurement is essential for meteorological monitoring, agricultural production, transportation, and other aspects of daily life and production. Furthermore, measuring high-speed wind speed and direction has significant practical implications in special applications, such as extreme weather conditions. Currently, widely used wind speed and direction sensors include mechanical, ultrasonic, and thermal types. Mechanical sensors suffer from significant wear and tear due to their movable mechanical components, resulting in high maintenance costs. Ultrasonic sensors offer high accuracy but are very expensive, limiting their widespread adoption. Thermal sensors utilize thermal convection with the wind to measure wind speed, have no moving parts, and can be mass-produced using MEMS technology, resulting in low cost and avoiding the aforementioned problems. However, thermal sensors, due to their inherent limitations, experience saturation in high-speed wind speed measurements. This is often mitigated by increasing power, but this higher power consumption contradicts the low-power advantage of MEMS devices, limiting the measurement range of thermal sensors.

[0003] A capacitor is generally defined as a device that stores opposite charges between two conductors. When the spacing and relative position of the capacitor change due to an external excitation, its capacitance changes accordingly. This sensing mechanism allows it to be used as a sensor. A parallel-plate capacitor consists of two conducting plates parallel to each other in the width direction. More broadly, this also includes plates that are not strictly parallel or not planar. The capacitance of a parallel-plate capacitor is... Where ε is the dielectric constant between the two plates, A is the overlapping area of ​​the two plates, and the spacing is d. Combined with a cantilever beam, one plate is located at the bottom of the movable cantilever beam, and the other plate is located on the fixed substrate below. When a high-speed wind blows across the cantilever beam, the cantilever beam deforms under the force of the wind, causing changes in the spacing and overlapping area of ​​the capacitors. Measuring the capacitance change characterizes the high-speed wind speed information. The four capacitor structures are symmetrically distributed along the center of the chip, and the high-speed wind direction information can be obtained by combining these measurements. Furthermore, low-speed wind direction information is measured by four thermal resistors based on the thermal principle, realizing wind speed and direction measurement based on both thermal and capacitive methods, taking into account both low and high speeds. Summary of the Invention

[0004] Purpose of the invention: To address the limitations of existing wind speed sensors, a thermal-capacitive wide-range MEMS wind speed and direction sensor is proposed. This provides a new solution to the problem of limited range of thermal-based MEMS wind speed and direction sensors at high wind speeds. Furthermore, the combination of thermal and capacitive dual modes ensures the measurement accuracy of both high and low wind speeds within the specified range.

[0005] Technical solution:

[0006] A thermal-capacitive wide-range MEMS wind speed and direction sensor comprises two parts: a thermal resistance element and a capacitor. The sensor chip includes a thermal resistance element (1), a cantilever beam (2), a substrate (3), and a capacitor (4). The capacitor includes an upper plate (41) located at the bottom of the cantilever beam and a lower plate (42) located on the substrate. Several thermal resistance elements (1) are symmetrically distributed around the center of the chip on the top of the cantilever beam (2). The thermal resistance element (1) operates in a self-heating zone, forming a symmetrical temperature distribution field on the chip surface while detecting its own resistance. The upper plate (41) of the capacitor (4) is located at the bottom of the cantilever beam, forming a capacitor structure with the lower plate (42) located on the substrate (3). The cantilever beam deforms due to wind, thereby changing the capacitance value and measuring the wind speed. The thermal resistance element (1) is based on the thermal principle and is used to measure low-speed wind speed and direction; the capacitor element (4), including the upper plate (41) and the lower plate (42), is based on the capacitance principle and is used to measure high-speed wind speed and direction.

[0007] The cantilever beam structure has a thermal resistance element at the top and the upper plate of a capacitor element at the bottom. It serves both as a carrier for the element and as a wind-sensing component, deforming under wind force to change the capacitance and thus measure high-speed wind speed. In calm conditions, the cantilever beams exhibit a certain degree of warping, with each beam exhibiting the same degree of warping and each capacitor having the same capacitance value. Under higher wind speeds, the curvature of the upstream cantilever beam decreases while that of the downstream beam increases, resulting in a larger contact area and smaller distance between the plates of the upstream capacitor, thus altering the capacitance value. The upstream increases the capacitance, while the downstream decreases it. The difference between the upstream and downstream capacitance values ​​can represent the high-speed wind direction information. Combined with the y-direction component, the wind direction information can be obtained.

[0008] The fabrication method of a thermal-capacitive wide-range MEMS anemometer includes the following steps:

[0009] Step 1: Sputter metal onto the first silicon wafer and peel it off to form the lower electrode of the capacitor (41);

[0010] Step 2: Deposit, photolithographically etch, and etch metal or polysilicon on the front side of the second silicon wafer to form a thermal resistance element (1);

[0011] Step 3: Based on Step 2, flip the second silicon wafer and etch a square cavity structure on its back side;

[0012] Step 4: Based on step 3, sputter metal into the cavity structure and peel it off to form the upper electrode plate (42) of the capacitor;

[0013] Step 5: Based on step 4, the second silicon wafer is further etched to release the cantilever beam structure and retain some intrinsic stress, so that the cantilever beam has a certain degree of warping (similar effects can be achieved by using other materials and processes to form cantilever beams).

[0014] Step 6: Based on step 5, the first and second silicon wafers are thermally bonded with the capacitor plates facing each other to form a complete device structure.

[0015] Beneficial effects: (1) This invention adopts a combination of thermal and capacitive methods to achieve wide-range wind speed and direction measurement, while taking into account the measurement accuracy across the entire measurement range. In comparison, sensors that only use the thermal principle will not have a temperature difference between the upstream and downstream sides that is higher than the temperature difference between the chip and the environment at high wind speeds. Without sacrificing the power consumption advantage, it is impossible to achieve higher wind speed measurements. Sensors that only use a cantilever beam structure to measure high-speed wind speeds require the cantilever beam to have a certain strength to prevent breakage at high wind speeds, making it difficult to guarantee the measurement accuracy at low wind speeds. The use of the thermal-capacitive dual-mode can effectively avoid the above problems.

[0016] (2) This invention achieves low-speed wind speed measurement based on thermal resistance and operates in a self-heating mode, eliminating the need for additional heating resistors to provide a symmetrical thermal field. Compared with other principle-based sensors, it retains the advantages of thermal wind speed sensors. Furthermore, compared to other commonly used structures, such as designing heating resistors in the central area of ​​the chip or adding heating resistors around each of the four temperature-sensing resistors, this invention removes the limitations on heating resistor design, making the design more flexible and the structure simpler.

[0017] (3) This invention achieves high-speed wind speed measurement based on capacitors, and also realizes two-dimensional wind field detection through a symmetrically distributed structure. On the one hand, the capacitor structure makes up for the limitation of traditional thermal sensors that cannot measure high-speed wind speed, and significantly expands the measurement range; on the other hand, compared with other sensing principles, the electrostatic sensing principle adopted by the capacitor has a simple structure, requiring only two conductive surfaces, and does not require special functional materials, such as piezoresistive and piezoelectric sensing which require the deposition, photolithography and integration of special piezoresistive and piezoelectric materials.

[0018] (4) The thermal resistance element and the capacitor element of the thermal-capacitive dual-mode wind speed sensor proposed in this invention adopt a vertical structure and are integrated on the same cantilever beam, which expands the range of the device and simplifies the structure without increasing the chip area. Attached Figure Description

[0019] Figure 1 This is a three-dimensional schematic diagram of the MEMS wind speed and direction sensor chip structure of the present invention;

[0020] Figure 2This is a side view of the MEMS wind speed and direction sensor chip structure of the present invention;

[0021] Figure 3 This is a top view of the MEMS wind speed and direction sensor chip structure of the present invention.

[0022] In the attached image:

[0023] 1-Thermal resistance element (including R1-R4), 2-Cantilever beam, 3-Substrate, 41-Upper plate of capacitor element, 42-Lower plate of capacitor element, C1-C4 are capacitor elements. Detailed Implementation

[0024] The invention will now be further explained with reference to the accompanying drawings.

[0025] A thermal-capacitive wide-range MEMS wind speed and direction sensor includes a sensor chip comprising a thermal resistance element 1, a cantilever beam structure 2, a substrate 3, and a capacitor element 4. The cantilever beam structure 2 has the thermal resistance element 1 at its top and the upper electrode 41 of the capacitor element at its bottom. It serves both as a carrier of the element and as a wind-sensing component, deforming under wind force to change the capacitance and thus measure high-speed wind speed. The thermal resistance element 1 is located at the top of the cantilever beam structure 2 and symmetrically distributed along the center of the chip. It operates in a self-heating zone, providing heat to raise the chip temperature above the ambient temperature while simultaneously measuring the temperature gradient caused by asymmetric heat transfer from the wind, thus measuring low-speed wind direction. The upper electrode 41 of the capacitor structure is located at the bottom of the cantilever beam, and the lower electrode 42 is fixed to the substrate, maintaining a centrally symmetrical distribution along the chip center. Under high wind speeds, the cantilever beam 2 deforms under wind force, changing the distance and overlap area between the capacitor plates. The change in capacitance is used to measure high-speed wind speed, and the four capacitor structures together complete the wind direction measurement.

[0026] When the wind speed and direction sensor of this invention is working, it adopts thermal and capacitive measurement principles for different wind speed stages, but the principle is the same in the x and y directions, and the x-direction is taken as an example. In the low wind speed stage, it is based on the thermal principle: the four thermistors work in self-heating mode. When there is no wind, the heat provided by the four resistors is the same, forming a symmetrically distributed thermal field on the chip surface, and there is no temperature difference signal output. When there is wind, the temperature of the chip surface drops due to natural convection heat transfer and forced convection heat transfer. In the x direction, thermal convection will take away more heat from the upstream, making the temperature of the upstream resistor slightly lower than that of the downstream resistor, generating a temperature gradient. The wind speed information can be measured by the resistance change of the thermistors of the two resistors. By integrating the temperature gradients in the x and y directions, the wind direction information can be obtained. During high wind speeds, based on the capacitor principle: the upper plate 41 of the capacitor is located at the bottom of the cantilever beam, and the lower plate 42 is fixed on the substrate below it. In the absence of wind, the cantilever beam warps to a certain extent, with all four cantilever beams warping to the same degree, resulting in all four capacitors having the same capacitance value. However, under higher wind speeds, the curvature of the upstream cantilever beam decreases while that of the downstream beam increases, leading to an increase in the contact area and decrease in the distance between the two plates of the upstream capacitor, thus increasing the capacitance value. The upstream increases the capacitance, while the downstream decreases it. The difference between the upstream and downstream capacitance values ​​can represent the high-speed wind direction information. Combined with the y-direction component, the wind direction information can be obtained.

[0027] A method for fabricating a thermal-capacitive wide-range MEMS anemometer includes the following steps:

[0028] Step 1: Sputter metal onto the first silicon wafer and peel it off to form the lower electrode 41 of the capacitor;

[0029] Step 2: Deposit, photolithographically etch, and etch metal or polysilicon on the front side of the second silicon wafer to form thermal resistance element 1;

[0030] Step 3: Based on Step 2, flip the second silicon wafer and etch a square cavity structure on its back side;

[0031] Step 4: Based on step 3, sputter metal into the cavity structure and peel it off to form the upper electrode 42 of the capacitor;

[0032] Step 5: Based on step 4, the second silicon wafer is further etched to release the cantilever beam structure and retain some intrinsic stress, so that the cantilever beam has a certain degree of warping (similar effects can be achieved by using other materials and processes to form cantilever beams).

[0033] Step 6: Based on step 5, the first and second silicon wafers are thermally bonded with the capacitor plates facing each other to form a complete device structure.

[0034] This invention is based on thermal resistance and capacitors. By reading the thermal resistance in four directions at low wind speeds and the capacitance in four directions at high wind speeds, it can achieve the measurement of wind direction compatible with both low and high wind speeds.

[0035] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for fabricating a thermal-capacitive wide-range MEMS anemometer, characterized in that, The sensor chip includes a thermal resistance element (1), a cantilever beam (2), a substrate (3), and a capacitor (4); the capacitor (4) includes an upper electrode plate (41) located at the bottom of the cantilever beam (2) and a lower electrode plate (42) located on the substrate (3); the thermal resistance element (1) is distributed at the top of the cantilever beam (2); the cantilever beam (2) has a certain degree of warping, and the degree of warping of each cantilever beam (2) is the same; the thermal resistance element (1), the cantilever beam (2), and the capacitor (4) are symmetrically distributed around the center of the chip. During operation, different wind speed stages employ thermal and capacitive measurement principles respectively, but the principles are consistent in the x and y directions. In the low wind speed stage, the thermal principle is used: the four thermistors operate in self-heating mode. When there is no wind, the four resistors provide the same amount of heat, forming a symmetrically distributed thermal field on the chip surface, with no temperature difference signal output. When there is wind, the chip surface temperature decreases due to natural convection and forced convection heat transfer. Furthermore, along the x direction, thermal convection carries away more heat from the upstream, causing the upstream resistor temperature to be slightly lower than the downstream resistor, creating a temperature gradient. Wind speed information can be measured by the resistance change of the two thermistors. By integrating the temperature gradients in the x and y directions, wind direction information can be obtained. In the high wind speed stage, based on the capacitor principle: the upper plate (41) of the capacitor is located at the bottom of the cantilever beam, and the lower plate (42) is fixed on the substrate below it. When there is no wind, the cantilever beam has a certain degree of warping. The four cantilever beams have the same degree of warping, and the four capacitors have the same capacitance value. When there is a high wind speed, under the action of wind, the arc of the upstream cantilever beam decreases, while that of the downstream increases, which makes the contact area between the two plates of the upstream capacitor increase and the distance decrease, and the capacitance value increases. The opposite is true for the downstream, where the capacitance value decreases. A difference appears between the capacitance values ​​of the upstream and downstream, which can characterize the high-speed wind direction information. Combined with the y-direction component, the wind direction information is obtained. The method for preparing the sensor includes the following steps: Step 1: Sputter metal onto the first silicon wafer and peel it off to form the lower electrode of the capacitor (42). Step 2: Deposit, photolithographically etch and etch metal or polysilicon on the front side of the second silicon wafer to form a thermal resistance element (1); Step 3: Based on Step 2, flip the second silicon wafer and etch a square cavity structure on its back side; Step 4: Based on step 3, sputter metal into the cavity structure and peel it off to form the upper electrode plate (41) of the capacitor. Step 5: Based on Step 4, form a cantilever beam on the second silicon wafer; Step 6: Based on step 5, the first and second silicon wafers are thermally bonded with the capacitor plates facing each other to form a complete device structure.

2. The preparation method according to claim 1, characterized in that, The thermal resistance element (1) operates in the self-heating zone, and detects its own resistance while forming a symmetrical temperature distribution field on the chip surface.

3. The preparation method according to claim 1, characterized in that, In step 5, the method for forming the cantilever beam is as follows: further etching is performed on the second silicon wafer to release the cantilever beam structure and retain a portion of the intrinsic stress, so that the cantilever beam has a certain degree of warping.

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