Monolithic integrated multi-frequency CMUTs for wide-range pressure measurement

Abstract

This invention relates to the field of pressure (0~200 kPa) detection, and more particularly to MEMS integrated pressure sensor devices or monolithically integrated multi-frequency capacitive micromechanical ultrasonic transducers (CMUTs) for wide-range pressure measurement. The monolithically integrated multi-frequency CMUTs integrate a first CMUT, a second CMUT, and a third CMUT on a single chip. By adjusting the diaphragm thickness, cavity height, diaphragm radius of the first, second, and third CMUT units, as well as the pressure value within the cavity of the second CMUT unit, the first and second CMUT units operate within a linear range of small deflection deformation, and the third CMUT unit operates within a linear range of large deflection deformation. Furthermore, the operating ranges of the first, second, and third CMUT units constitute a continuous and complete measurement interval, with resonant frequencies that do not overlap, thus realizing an integrated pressure sensor device usable for measuring the entire pressure range of 0~200 kPa.

Description

Technical Field

[0001] This invention relates to the field of pressure (0~200kPa) detection, and more particularly to MEMS integrated pressure sensor devices or monolithic integrated multi-frequency capacitive micromechanical ultrasonic transducers (CMUTs) for wide-range pressure measurement. Background Technology

[0002] Precision pressure measurement has long been a pressing technical challenge in fields such as industrial automation, aerospace, biomedicine, precision agriculture, environmental monitoring, and consumer electronics. Traditional pressure sensors, due to their large size, poor performance, high power consumption, complex structure, and high cost, can no longer meet increasingly complex and diverse needs, significantly hindering the normal development of related fields. MEMS-based pressure sensors, with their unique advantages of low cost, high performance, and miniaturization, have become an important research and application hotspot. MEMS pressure sensors are mainly divided into piezoresistive, capacitive, and resonant pressure sensors. Among them, the inherent temperature characteristics of piezoresistive MEMS pressure sensors have a significant impact on measurement accuracy, and necessary temperature compensation greatly limits their application range. The nonlinear characteristics and small output capacitance of capacitive MEMS pressure sensors place high demands on the detection signal, posing a significant challenge to the design of signal acquisition circuits. In contrast, resonant MEMS pressure sensors are mainly affected by the mechanical characteristics of the device structure itself, exhibiting good stability and low hysteresis, thus becoming a strong candidate for developing high-performance pressure sensors.

[0003] As a type of resonant MEMS pressure sensor, the capacitive micromechanical ultrasonic transducer (CMUT) typically has a thin and lightweight vibrating plate, resulting in low mechanical impedance. The vacuum gap allowed in CMUTs improves electromechanical conversion efficiency and reliability against electrical breakdown. These structural features contribute to the excellent resonant characteristics of CMUTs. Furthermore, CMUTs often consist of multi-resonator structures with hundreds or thousands of resonators connected in parallel, offering enhanced reliability compared to single-resonator systems, a wide impedance range for matching electronics, and low-frequency noise in the oscillation circuitry. CMUT devices require advanced MEMS fabrication technologies; relatively mature manufacturing methods and materials not only allow for good prediction of device characteristics but also provide greater design flexibility.

[0004] Currently, research and implementation of CMUT pressure sensors are relatively limited. Most CMUTs used for measuring pressures in the 0–200 kPa range yield results similar to those shown in Figure 1. Firstly, the resonant frequency of the CMUTs initially decreases and then increases with increasing pressure, due to the combined effects of spring softening and stress stiffening. When the CMUT diaphragm is in a small deflection state, the spring softening effect dominates, and the resonant frequency continuously decreases with increasing pressure. Conversely, when the CMUT diaphragm is in a large deflection state, the stress stiffening effect dominates, and the resonant frequency continuously increases with increasing pressure. Typically, when the diaphragm deflection is much smaller than its thickness (or the ratio of deflection to thickness is greater than 20), it is considered a small deflection deformation. Secondly, most frequency-pressure relationship curves are nonlinear, meaning that the actual pressure range that CMUTs can accurately measure is very limited, restricted to 0~50 kPa (negative pressure zone) and 150~200 kPa (positive pressure zone; 0~100 kPa is defined as the negative pressure zone, and 100~200 kPa as the positive pressure zone). This range is even smaller, and the sensitivity and linearity are low. Thirdly, the resonant frequencies of CMUTs in the 0~50 kPa and 150~200 kPa ranges partially overlap. If the pressure is measured using only the same sensitive element, it can easily lead to pressure identification errors. Although some work uses structural design to make CMUTs operate with minimal deflection deformation (…),… Figure 2 ) or large deflection deformation ( Figure 3 In the region of large deflection deformation, however, CMUTs with small deflection deformation working mechanism will have lower sensitivity to achieve wide range pressure measurement, and CMUTs with large deflection deformation working mechanism will still inevitably have obvious nonlinearity. Summary of the Invention

[0005] As described in the background section, existing CMUT pressure sensor devices suffer from the following technical problems: the pressure measurement range of a single sensing element design is discontinuous, with a large nonlinear region and frequency overlap; under small deflection deformation operating mechanisms, the sensitivity of a single sensing element design for wide-range pressure measurement is very low; and under large deflection deformation operating mechanisms, a single sensing element design always exhibits a significant nonlinear region. To solve these technical problems, this invention provides a monolithic integrated multi-frequency CMUT for wide-range pressure measurement.

[0006] The present invention discloses a CMUT unit with adjustable internal cavity pressure, comprising a silicon substrate as the bottom electrode and etched with a cavity, a silicon dioxide and single crystal silicon laminated film suspended above the cavity, and a metal film deposited above the laminated film as the top electrode; the cavity is filled with a gas at a certain pressure, wherein the certain pressure refers to a pressure control value in the range of 0~50 kPa.

[0007] This invention, based on a typical CMUT unit with a vacuum cavity structure, adjusts the pressure within the cavity of the CMUT unit to a certain level. When the CMUT unit detects under pressure exceeding the pressure control value, the pressures inside and outside the cavity cancel each other out. Therefore, the net pressure actually borne by the CMUT diaphragm is the external pressure minus the pressure control value, effectively allowing the CMUT unit to operate in a lower pressure region, thus improving the pressure sensitivity and linearity of the CMUT unit within the measured pressure range. Alternatively, depending on the specific application, the cavity pressure can be adjusted to any pressure control value within the range of 0-50 kPa, allowing the CMUT unit to operate within a preset measurement range while maintaining good pressure sensitivity and linearity.

[0008] Furthermore, ventilation holes are formed on the silicon dioxide and single-crystal silicon laminate film by RIE (reactive ion etching), and the ventilation holes are sealed by PECVD (plasma-enhanced chemical vapor deposition) under a certain pressure to achieve an internal cavity with a certain pressure.

[0009] This process allows the CMUT unit to be filled with gas at a certain pressure, so that the pressure difference between the inside and outside of the CMUT unit cavity is lower than the pressure control value, which is equivalent to operating in the region between 0 kPa and the pressure control value.

[0010] The monolithically integrated multi-frequency CMUTs for wide-range pressure measurement described in this invention integrate at least one first CMUT unit, at least one second CMUT unit, and at least one third CMUT unit on a chip; the second CMUT unit adopts a CMUT unit with adjustable internal cavity pressure (the first and third CMUT units adopt typical CMUT units with traditional vacuum cavity structure).

[0011] By adjusting the diaphragm thickness, cavity height, diaphragm radius of the first, second, and third CMUT units, as well as the pressure value inside the cavity of the second CMUT unit, the first and second CMUT units are made to work within the linear range of small deflection deformation, and the third CMUT unit is made to work within the linear range of large deflection deformation. Furthermore, the working ranges of the first, second, and third CMUT units constitute a continuous and complete measurement interval, and their resonant frequencies do not overlap.

[0012] The diaphragm thickness refers to the sum of the thicknesses of the silicon dioxide and monocrystalline silicon laminated films, and the diaphragm radius is equal to the cavity radius.

[0013] By adjusting the gas pressure inside the second CMUT unit, combined with adjustments to the diaphragm thickness, cavity height, and diaphragm radius, both the first and second CMUT units can operate within a linear range with a continuous measurement interval. Simultaneously, the third CMUT unit can operate within a linear range with a measurement interval continuous with that of the second CMUT unit. This invention integrates three CMUT units with different operating frequencies onto a single chip, achieving frequency division across different pressure ranges and ensuring a continuous and complete measurement interval. This overcomes the technical problems of existing single-sensor unit designs, such as the persistent significant nonlinearity.

[0014] Furthermore, the diaphragm thickness of the first, second, and third CMUT units is 5µm, the cavity height is 3µm, and the diaphragm radii are 120µm, 125µm, and 200µm, respectively. The CMUT unit with adjustable internal cavity pressure is filled with gas at a pressure of 50 kPa. The first CMUT unit operates at 0~50 kPa, the second CMUT unit operates at 50~100 kPa, and the third CMUT unit operates at 100~200 kPa.

[0015] Furthermore, multiple first CMUT units are arranged in a matrix to form first CMUTs (first capacitive micromechanical ultrasonic transducer array elements), multiple second CMUT units are arranged in a matrix to form second CMUTs (second capacitive micromechanical ultrasonic transducer array elements), and multiple third CMUT units are arranged in a matrix to form third CMUTs (third capacitive micromechanical ultrasonic transducer array elements); the three capacitive micromechanical ultrasonic transducer array elements constitute a monolithic integrated multi-frequency capacitive micromechanical ultrasonic transducer array.

[0016] The metal film is aluminum.

[0017] The beneficial effects of the present invention compared with the prior art are: (1) Compared with the segmented linear regions of traditional CMUTs with a single sensitive unit design of 0~50 kPa and 150~200 kPa, the effective pressure sensing region of the monolithic integrated multi-frequency CMUTs of the present invention is 0~200 kPa, and the resonant frequencies do not overlap at all.

[0018] (2) Compared with small deflection deformation CMUTs designed with a single sensitive unit, the monolithic integrated CMUTs of the present invention improve the pressure sensitivity by 124%, 125% and 4500% in the ranges of 0~50 kPa, 50~100 kPa and 100~200 kPa, respectively.

[0019] (3) Compared with traditional CMUTs with a single sensitive unit design and a cavity pressure of 0 kPa, the pressure sensitivity and linearity of CMUTs with a measurement range of 50~100 kPa proposed in this invention are improved by 254% and 12%, respectively, by adjusting the internal pressure of the cavity to a predetermined value (50 kPa).

[0020] (4) Compared with large deflection deformation CMUTs designed with a single sensitive unit, the present invention has no obvious nonlinear region. Attached Figure Description

[0021] Figure 1 A schematic diagram of measurement results for pressures in the 0–200 kPa range when CMUTs are designed with a single sensing element.

[0022] Figure 2 A schematic diagram of measurement results when CMUTs are designed with a single sensing element, by reducing the diaphragm radius so that the CMUTs always operate under a small deflection deformation mechanism.

[0023] Figure 3 A schematic diagram showing the measurement results when CMUTs are designed with a single sensing element, by increasing the diaphragm radius to make more of the CMUTs fall within the linear range of large deflection deformation.

[0024] Figure 4 A schematic diagram of the CMUT unit structure described in this invention.

[0025] Figure 5 This invention presents a schematic diagram of the structure of integrating CMUTs with different operating frequencies onto a single chip.

[0026] Figure 6 The CMUTs described in this invention exhibit pressure sensitivity and linearity in the range of 50~100 kPa.

[0027] Figure 7 The CMUTs described in this invention exhibit pressure sensitivity and linearity in the range of 0~200 kPa.

[0028] 1-Metal thin film, 2-Silicon dioxide, 3-Single crystal silicon, 4-Silicon substrate, 5-First CMUT unit, 6-Second CMUT unit, 7-Third CMUT unit. Detailed Implementation

[0029] Example 1: A CMUT unit with adjustable internal cavity pressure includes a silicon substrate with an etched cavity as the bottom electrode, a silicon dioxide and monocrystalline silicon laminated film suspended above the cavity, and a metal film deposited above the laminated film as the top electrode. The cavity is filled with gas at a certain pressure, wherein the certain pressure refers to a pressure control value within the range of 0~50 kPa. Vent holes are formed on the silicon dioxide and monocrystalline silicon laminated film by RIE etching, and the vent holes are sealed by PECVD silicon dioxide under a certain pressure to achieve an internal cavity with a certain pressure.

[0030] Example 2: A monolithically integrated multi-frequency CMUTs for wide-range pressure measurement, comprising at least one first CMUT unit, at least one second CMUT unit, and at least one third CMUT unit integrated on a chip; the second CMUT unit adopts the cavity-adjustable internal pressure CMUT unit described in Example 1; by adjusting the diaphragm thickness, cavity height, diaphragm radius of the first, second, and third CMUT units, as well as the pressure value inside the cavity of the second CMUT unit, the first and second CMUT units operate within the linear range of small deflection deformation, and the third CMUT unit operates within the linear range of large deflection deformation, and the operating ranges of the first, second, and third CMUT units constitute a continuous and complete measurement interval, with resonant frequencies that do not overlap.

[0031] Example 3: Based on Example 2, the diaphragm thickness of the first, second, and third CMUT units is 5µm, the cavity height is 3µm, and the diaphragm radii are 120µm, 125µm, and 200µm, respectively; the CMUT unit with adjustable internal cavity pressure is filled with gas at a pressure of 50 kPa.

[0032] The following description, in conjunction with the accompanying drawings, further explains the inventive basis and concept of the present invention.

[0033] This invention adopts Figure 4 The CMUT unit structure shown is illustrated, and three CMUTs with different operating frequencies are designed and implemented using structural parameters (diaphragm radius and cavity height) for pressure measurement in different ranges: 0~50 kPa, 50~100 kPa, and 100~200 kPa. Their resonant frequencies do not overlap. The unit structure parameters for the three different CMUTs are shown in Table 1. This invention employs... Figure 5 The distribution shown integrates CMUTs with different operating frequencies onto a single chip, achieving frequency division for different pressure ranges.

[0034] .

[0035] The CMUTs used for pressure detection in the 0~50 kPa and 50~100 kPa ranges operate under a small deflection deformation mechanism, while those used for pressure detection in the 100~200 kPa range operate under a large deflection deformation mechanism. Specifically, the CMUTs used for pressure detection in the 50~100 kPa range exhibit low linearity and sensitivity for each CMUT unit after manufacturing using conventional CMUT processes. This invention addresses this by forming venting holes on a silicon thin film through RIE etching and sealing these holes with PECVD silicon dioxide under a pressure of 50 kPa. By adding an additional process step, the pressure inside the CMUT unit cavity is adjusted from P1=0 kPa to P2=50 kPa. When CMUTs with CMUT units arranged to have a cavity pressure of 50 kPa are used for pressure detection in the 50~100 kPa range, the pressures inside and outside the cavity cancel each other out; therefore, the actual net pressure borne by the CMUT diaphragm is 0~50 kPa. This is equivalent to CMUTs with P1=0 kPa operating in a pressure range of 0~50 kPa, thus improving the pressure sensitivity and linearity of CMUTs in the 50~100 kPa range, such as... Figure 6 As shown. The cavity pressures of the CMUTs used for pressure testing in the ranges of 0~50 kPa, 50~100 kPa, and 100~200 kPa are 0 kPa, 50 kPa, and 0 kPa, respectively. Furthermore, while ensuring good measurement linearity within the 0~50 kPa pressure range, this invention improves the pressure sensitivity of the CMUTs by maximizing the diaphragm radius. All CMUTs are arranged in a matrix distribution. Figure 5 The CMUT units in each CMUTs are arranged in a 3×3 matrix, but a larger matrix arrangement can also be used.

[0036] This invention eliminates the nonlinear region in the 0-200 kPa range through multi-frequency CMUTs design and additional process steps, achieving full coverage of the effective sensing area and forming an integrated pressure sensor device with high sensitivity and linearity, such as... Figure 7 As shown. In practical applications, CMUTs use multi-channel signal output, and the signal of a certain channel is selected according to actual needs.

[0037] As can be seen from the accompanying drawings, the present invention has the following significant technical effects compared with the prior art: (1) Compared with Figure 1 Traditional CMUTs with a single sensing unit have piecewise linear pressure sensing regions of 0-50 kPa and 150-200 kPa, or even smaller, with most of the resonant frequencies overlapping in these regions. The monolithically integrated multi-frequency CMUTs of this invention have an effective pressure sensing region of 0-200 kPa, and their resonant frequencies do not overlap at all. Figure 7.

[0038] (2) Compared to Figure 2 The small deflection deformation CMUTs with a single sensitive unit design have a pressure measurement sensitivity of 12.25 Hz / kPa in the range of 0~200 kPa; the monolithic integrated CMUTs of the present invention have pressure sensitivities of 27.49 Hz / kPa, 27.53 Hz / kPa and 563.44 Hz / kPa in the ranges of 0~50 kPa, 50~100 kPa and 100~200 kPa respectively, which are improvements of 124%, 125% and 4500% respectively.

[0039] (3) such as Figure 6 As shown, the pressure sensitivity and linearity of CMUTs with a cavity pressure of 0 kPa are 7.77 Hz / kPa and 0.88742, respectively. By adjusting the internal cavity pressure to a predetermined value (50 kPa) as proposed in this invention, the pressure sensitivity and linearity of CMUTs in the range of 50~100 kPa are improved to 27.53 Hz / kPa and 0.99014, respectively, representing improvements of 254% and 12%.

[0040] (4) Compared to Figure 3 Large-deflection CMUTs with a single sensitive element design exhibit a significant nonlinear region in the 0-50 kPa range, or even a wider range; this invention, however, shows no significant nonlinear region and is evident in… Figure 7 .

[0041] In addition to the structure designed in this invention, solutions for achieving high-performance integrated pressure sensors through multi-frequency CMUTs and monolithic integration are all within the scope of innovation claimed in this invention.

Claims

1. A monolithic integrated multi-frequency CMUTs for wide-range pressure measurement, characterized in that, The chip integrates at least one first CMUT unit, at least one second CMUT unit, and at least one third CMUT unit; the second CMUT unit is a CMUT unit with adjustable internal cavity pressure; the CMUT unit with adjustable internal cavity pressure includes a silicon substrate as the bottom electrode and etched with a cavity, a silicon dioxide and single crystal silicon laminate film suspended above the cavity, and a metal film deposited above the laminate film as the top electrode; the cavity is filled with a gas at a certain pressure, the certain pressure being a pressure control value in the range of 0~50 kPa; By adjusting the diaphragm thickness, cavity height, diaphragm radius of the first, second, and third CMUT units, as well as the pressure control value inside the cavity of the second CMUT unit, the first and second CMUT units are made to work within the linear range of small deflection deformation, and the third CMUT unit is made to work within the linear range of large deflection deformation. Furthermore, the working ranges of the first, second, and third CMUT units constitute a continuous and complete measurement interval, and their resonant frequencies do not overlap. The diaphragm thickness refers to the sum of the thicknesses of the silicon dioxide and monocrystalline silicon laminated films, and the diaphragm radius is equal to the cavity radius. The diaphragm thickness of the first, second, and third CMUT units is 5µm, the cavity height is 3µm, and the diaphragm radii are 120µm, 125µm, and 200µm, respectively.

2. The monolithically integrated multi-frequency CMUTs for wide-range pressure measurement as described in claim 1, characterized in that, The CMUT unit with adjustable internal pressure has its cavity filled with gas at a pressure of 50 kPa.

3. The monolithically integrated multi-frequency CMUTs for wide-range pressure measurement as described in claim 1 or 2, characterized in that, The first, second, and third CMUTs consist of multiple units, arranged in a matrix, forming the first, second, and third CMUTs respectively.

4. The monolithically integrated multi-frequency CMUTs for wide-range pressure measurement as described in claim 1 or 2, characterized in that, The metal film is aluminum.

5. The monolithically integrated multi-frequency CMUTs for wide-range pressure measurement as described in claim 1, characterized in that, For CMUT units with adjustable internal cavity pressure, ventilation holes are formed on a silicon dioxide and monocrystalline silicon laminate film by RIE etching, and the ventilation holes are sealed by PECVD silicon dioxide under a certain pressure to achieve an internal cavity with a certain pressure.

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