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

Abstract

The invention relates to the field of pressure (0-200 kPa) detection, in particular to an MEMS integrated pressure sensing device or a monolithic integrated multi-frequency capacitive micromachined ultrasonic transducer (CMUTs) for wide-range pressure measurement. According to the monolithic integrated multi-frequency CMUTs, a first CMUTs, a second CMUTs and a third CMUTs are integrated on a chip; by adjusting the diaphragm thickness, cavity height and diaphragm radius of the first, second and third CMUT units and the pressure value in the cavity of the second CMUT unit, the first and second CMUT units work in a linear range of small deflection deformation, and the third CMUT unit works in a linear range of large deflection deformation. And the working ranges of the first CMUT unit, the second CMUT unit and the third CMUT unit form a continuous and complete measurement interval, and the resonant frequencies are not overlapped, so that the integrated pressure sensing device which can be used for 0-200 kPa full-pressure range measurement is realized.

Description

Technical Field

[0001] The present invention relates to the field of pressure (0-200 kPa) detection, and in particular to a MEMS integrated pressure sensor device or a monolithic integrated multi-frequency capacitive micromachined ultrasonic transducer (CMUTs) for wide-range pressure measurement. Background Art

[0002] Precision pressure measurement has always been a technical problem that urgently needs to be solved in the fields of industrial automation, aerospace, biomedicine, precision agriculture, environmental monitoring, consumer electronics, etc. Due to the shortcomings of large size, poor performance, high power consumption, complex structure, and high cost, traditional pressure sensors can no longer meet the increasingly complex and diversified needs, which greatly hinders the normal development process of related fields. Pressure sensors based on MEMS technology have become an important research and application hotspot with their unique advantages such as low cost, high performance and miniaturization. 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 great influence on the measurement accuracy, and the necessary temperature compensation will greatly limit the scope of application; the nonlinear characteristics and small output capacitance of capacitive MEMS pressure sensors have high requirements for detection signals, which brings great challenges to the design of signal pickup circuits. In comparison, resonant MEMS pressure sensors are mainly affected by the mechanical characteristics of the device structure itself, have good stability and low hysteresis, and therefore become a strong candidate for the development of high-performance pressure sensors.

[0003] As a type of resonant MEMS pressure sensor, the vibrating plate of the capacitive micromachined ultrasonic transducer (CMUT) is usually thin and light, so it has a low mechanical impedance; CMUT allows a vacuum gap, which is conducive to improving the electromechanical conversion efficiency and the reliability of resisting electrical breakdown. These structural features give CMUT good resonance characteristics. In addition, CMUTs are usually composed of a multi-resonator structure of hundreds or thousands of resonators connected in parallel, which can provide enhanced reliability over a single resonator system, a wide impedance range for matching electronic circuits, and low-frequency noise in the oscillation circuit. CMUT devices need to be implemented using advanced MEMS process technology. The relatively mature manufacturing methods and materials used not only allow good predictions of device characteristics, but also provide greater CMUT design flexibility.

[0004] At present, there are relatively few studies and implementations on CMUT pressure sensors. The measurement results of most CMUTs for pressures in the range of 0~200kPa are similar to those shown in Figure 1. First, the resonant frequency of CMUTs decreases first and then increases with increasing pressure, which is due to the combined effect of spring softening effect and stress stiffening effect. When the CMUT diaphragm is in a small deflection deformation state, the spring softening effect plays a dominant role, and the resonant frequency continues to decrease with increasing pressure; when the CMUT diaphragm is in a large deflection deformation state, the stress stiffening effect plays a dominant role, and the resonant frequency continues to increase with increasing pressure. Generally, when the diaphragm deflection is much smaller than the thickness (or the ratio of deflection to thickness is greater than 20), it is called small deflection deformation. Second, most frequency-pressure curves are nonlinear, which means that the pressure range that CMUTs can actually accurately measure is very limited, limited to 0~50 kPa (negative pressure zone) and 150~200 kPa (positive pressure zone; 0~100 kPa is defined as negative pressure zone, 100~200 kPa is defined as positive pressure zone). This range is even smaller, and the sensitivity and linearity are low; third, the resonant frequencies of CMUTs in the range of 0~50 kPa and 150~200 kPa partially overlap. If the pressure is measured only by the same sensitive element, it is easy to cause pressure identification errors. Although some work has been done to make CMUTs work as much as possible in the small deflection deformation ( Figure 2 ) or large deflection ( Figure 3 ) area, however, the CMUTs with small deflection deformation working mechanism will have lower sensitivity in achieving wide-range pressure measurement, and the CMUTs with large deflection deformation working mechanism will inevitably have obvious nonlinearity. Summary of the invention

[0005] It can be seen from the records in the background technology that the existing CMUT pressure sensor devices have the following technical problems: the pressure measurement range of the single sensitive element design is discontinuous, and the nonlinear region is large and the frequency overlap problem; under the small deflection deformation working mechanism, the sensitivity of the wide range pressure measurement that can be achieved by the single sensitive element design is very low, and under the large deflection deformation working mechanism, the single sensitive element design always has obvious nonlinear regions and other technical problems. In order to solve the above technical problems, the present invention provides a monolithic integrated multi-frequency CMUTs for wide range pressure measurement.

[0006] The present invention discloses a CMUT unit with adjustable cavity internal pressure, comprising a silicon substrate as a 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 a top electrode; the cavity is filled with a gas at a certain pressure, and the certain pressure refers to a pressure control value within the range of 0-50 kPa.

[0007] Based on the typical CMUT unit with a vacuum cavity structure, the present invention adjusts the cavity of the CMUT unit to have a certain pressure. When the CMUT unit is tested at a pressure greater than 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, which is equivalent to making the CMUT unit work in a lower pressure area, effectively improving the pressure sensitivity and linearity of the CMUT unit within the measured pressure range. Of course, in combination with actual application conditions, the cavity pressure can be adjusted to any pressure control value within the range of 0~50 kPa, so that the CMUT unit can work within the preset measurement range while maintaining good pressure sensitivity and linearity.

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

[0009] Through this process, the CMUT unit can be filled with gas at a certain pressure to achieve a pressure difference between the inside and outside of the cavity of the CMUT unit lower than the pressure control value, which is equivalent to working in the area between 0 kPa and the pressure control value.

[0010] The monolithic integrated multi-frequency CMUTs for wide-range pressure measurement of the present invention integrates at least one first CMUT unit, at least one second CMUT unit and at least one third CMUT unit on the chip; the second CMUT unit adopts a CMUT unit with adjustable internal pressure of the cavity (the first and third CMUT units adopt typical CMUT units with traditional vacuum cavity structure); By adjusting the diaphragm thickness, cavity height, diaphragm radius of the first, second and third CMUT units and the pressure value in the cavity of the second CMUT unit, the first and second CMUT units are operated within the linear range of small deflection deformation, the third CMUT unit is operated 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, and the resonant frequencies do not overlap each other; The diaphragm thickness refers to the sum of the thickness of the silicon dioxide and single crystal silicon laminated films, and the diaphragm radius is equal to the cavity radius.

[0011] By adjusting the pressure value of the gas inside the second CMUT unit, combined with the adjustment of the diaphragm thickness, cavity height, and diaphragm radius, the first and second CMUT units can be made to work within the linear range and the measurement interval can be kept continuous, while the third CMUT unit can be made to work within the linear range and the measurement interval can also be kept continuous with the second CMUT unit. The present invention integrates the above three CMUT units with different operating frequencies on one chip, realizes the frequency division of different pressure ranges, and the measurement interval is continuous and complete, overcoming the technical problems of the single sensitive unit design in the prior art, such as the obvious nonlinear region.

[0012] Furthermore, the diaphragm thickness of the first, second and third CMUT units is 5µm, the cavity height is 3µm, and the diaphragm radius is 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 works at 0~50 kPa, the second CMUT unit works at 50~100 kPa, and the third CMUT unit works at 100~200 kPa.

[0013] Furthermore, multiple first CMUT units are arranged in a matrix to constitute first CMUTs (first capacitive micromachined ultrasonic transducer array elements), multiple second CMUT units are arranged in a matrix to constitute second CMUTs (second capacitive micromachined ultrasonic transducer array elements), and multiple third CMUT units are arranged in a matrix to constitute third CMUTs (third capacitive micromachined ultrasonic transducer array elements); three capacitive micromachined ultrasonic transducer array elements constitute a monolithic integrated multi-frequency capacitive micromachined ultrasonic transducer array.

[0014] The metal film is aluminum.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) Compared with the conventional CMUTs with a single sensitive unit design having a piecewise linear region 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.

[0016] (2) Compared with the small deflection deformation CMUTs designed with a single sensitive unit, the pressure sensitivity of the monolithic integrated CMUTs of the present invention in the ranges of 0~50 kPa, 50~100 kPa and 100~200 kPa is improved by 124%, 125% and 4500%, respectively.

[0017] (3) Compared with the traditional CMUTs with a single sensitive unit design and a cavity pressure of 0 kPa, the pressure sensitivity and linearity of the CMUTs proposed in the present invention, which adjust the internal cavity pressure to a predetermined value (50 kPa) in the measurement range of 50~100 kPa, are improved by 254% and 12%, respectively.

[0018] (4) Compared with large deflection CMUTs designed with a single sensitive unit, the present invention has no obvious nonlinear region. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the measurement results for pressures in the range of 0 to 200 kPa when CMUTs are designed with a single sensitive unit.

[0020] Figure 2 Schematic diagram of the measurement results when CMUTs are designed with a single sensitive unit and the diaphragm radius is reduced so that the CMUTs always work under the small deflection deformation mechanism.

[0021] Figure 3 Schematic diagram of the measurement results of increasing the diaphragm radius to make a larger proportion of CMUTs in the linear range of large deflection deformation when CMUTs adopt a single sensitive unit design.

[0022] Figure 4 Schematic diagram of the CMUT unit structure of the present invention.

[0023] Figure 5 A schematic diagram of the structure of the present invention integrating CMUTs with different operating frequencies on a single chip.

[0024] Figure 6 The pressure sensitivity and linearity of the CMUTs described in the present invention are in the range of 50-100 kPa.

[0025] Figure 7 The pressure sensitivity and linearity of the CMUTs described in the present invention are in the range of 0~200 kPa.

[0026] 1-metal film, 2-silicon dioxide, 3-single crystal silicon, 4-silicon substrate, 5-first CMUT unit, 6-second CMUT unit, 7-third CMUT unit. DETAILED DESCRIPTION

[0027] Embodiment 1 A CMUT unit with adjustable cavity internal pressure includes a silicon substrate as a 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 a top electrode; the cavity is filled with a gas at a certain pressure, and the certain pressure refers to a pressure control value within the range of 0 to 50 kPa. Vents are formed on the silicon dioxide and single crystal silicon laminate film by RIE etching, and the vents are sealed by PECVD silicon dioxide under a certain pressure to achieve a cavity with a certain pressure inside.

[0028] Embodiment 2 A monolithic 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 the chip; the second CMUT unit adopts the CMUT unit with adjustable internal cavity pressure as described in Embodiment 1; by adjusting the diaphragm thickness, cavity height, diaphragm radius of the first, second and third CMUT units and the pressure value in the cavity of the second CMUT unit, the first and second CMUT units are made to operate within the linear range of small deflection deformation, the third CMUT unit is made to operate within the linear range of large deflection deformation, and the operating range of the first, second and third CMUT units constitutes a continuous and complete measurement interval, and the resonant frequencies do not overlap.

[0029] 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 radius is 120µm, 125µm and 200µm respectively; the CMUT unit with adjustable internal cavity pressure is filled with gas at a pressure of 50 kPa.

[0030] The invention cause and inventive concept of the present invention are further described below in conjunction with the accompanying drawings.

[0031] The present invention adopts Figure 4 The CMUT unit structure shown in the figure is used to design and realize CMUTs with three different operating frequencies through structural parameters (diaphragm radius and cavity height), which are used for pressure measurement in different ranges of 0~50 kPa, 50~100 kPa and 100~200 kPa, and their resonant frequencies do not overlap with each other. The unit structure parameters of the three different CMUTs can be seen in Table 1. Figure 5 The distribution shown integrates CMUTs with different operating frequencies on a single chip, achieving frequency division for different pressure ranges.

[0032] .

[0033] Among them, the CMUTs used for pressure detection in the range of 0~50 kPa and 50~100 kPa work in a small deflection deformation mechanism, and the CMUTs used for pressure detection in the range of 100~200 kPa work in a large deflection deformation mechanism. Among them, the CMUTs used for pressure detection in the range of 50~100 kPa, each CMUT unit therein has low linearity and sensitivity after being manufactured using the traditional CMUT process flow. The present invention forms a vent on a silicon film by RIE etching, and seals the vent by PECVD silicon dioxide at a pressure of 50 kPa, and adjusts the pressure in the cavity of the CMUT unit from P1=0 kPa to P2=50 kPa by adding additional process steps. When the CMUTs arranged with CMUT units with a cavity pressure of 50 kPa are used for pressure detection in the range of 50~100 kPa, the pressures inside and outside the cavity offset each other, so the net pressure actually borne by the CMUTs diaphragm is 0~50 kPa. This is equivalent to the CMUTs with P1 = 0 kPa working in the pressure range of 0~50 kPa, and thus improves the pressure sensitivity and linearity of the CMUTs in the range of 50~100 kPa, such as Figure 6 As shown. The pressure in the cavity of the CMUTs used for the pressure test in the range of 0-50 kPa, 50-100 kPa and 100-200 kPa is 0 kPa, 50 kPa and 0 kPa respectively. In addition, under the premise of ensuring good measurement linearity in the pressure range of 0-50 kPa, the present invention improves the pressure sensitivity of the CMUTs by increasing the diaphragm radius as much as possible. The CMUTs are all distributed in a matrix. Figure 5 The CMUT units in each CMUTs are arranged in a 3´3 matrix, although a larger matrix arrangement may also be used.

[0034] The present invention eliminates the nonlinear region in the range of 0-200 kPa through multi-frequency CMUTs design and additional process steps, achieves full coverage of the effective sensing area, and forms an integrated pressure sensor device with high sensitivity and linearity, such as Figure 7 In actual applications, CMUTs use multi-channel signal outputs and decide to use a certain channel signal based on actual needs.

[0035] Combined with the accompanying drawings, it can be seen that the present invention has the following significant technical effects compared with the prior art: (1) Compared with Figure 1 The conventional CMUTs with a single sensitive unit have a piecewise linear region of 0-50 kPa and 150-200 kPa, or even a smaller range, and the resonance frequencies of these regions mostly overlap; the effective pressure sensing region of the monolithic integrated multi-frequency CMUTs of the present invention is 0-200 kPa, and the resonance frequencies do not overlap at all, as can be seen in Figure 7.

[0036] (2) Compared with Figure 2 The small deflection deformation CMUTs designed with a single sensitive unit have a pressure measurement sensitivity of 12.25 Hz / kPa in the range of 0~200 kPa; the pressure sensitivities of the monolithic integrated CMUTs of the present invention in the ranges of 0~50 kPa, 50~100 kPa and 100~200 kPa are 27.49 Hz / kPa, 27.53 Hz / kPa and 563.44 Hz / kPa, respectively, which are increased by 124%, 125% and 4500%, respectively.

[0037] (3) If Figure 6 As shown, the pressure sensitivity and linearity of the CMUTs with a cavity pressure of 0 kPa are 7.77 Hz / kPa and 0.88742, respectively; by adjusting the internal pressure of the cavity to a predetermined value (50 kPa) as proposed in the present invention, the pressure sensitivity and linearity of the CMUTs in the range of 50-100 kPa are increased to 27.53 Hz / kPa and 0.99014, respectively, which are increased by 254% and 12%, respectively.

[0038] (4) Compared with Figure 3 The large deflection deformation CMUTs designed with a single sensitive unit have obvious nonlinear regions in the range of 0~50 kPa, and even a larger range; the present invention has no obvious nonlinear region, which can be seen in Figure 7 .

[0039] In addition to the structure designed by the present invention, solutions for realizing high-performance integrated pressure sensors through multi-frequency CMUTs and monolithic integration are all within the scope of innovation claimed for protection by the present invention.

Claims

1. A CMUT unit with adjustable cavity internal pressure, comprising a silicon substrate with a cavity etched therein as a bottom electrode, a silicon dioxide and single crystal silicon laminate film suspended above the cavity, and a metal film deposited above the laminate film as a top electrode; characterized in that: The cavity is filled with gas at a certain pressure, and the certain pressure refers to a pressure control value within the range of 0~50 kPa.

2. The CMUT unit with adjustable cavity internal pressure as claimed in claim 1, characterized in that: Vents are formed on the silicon dioxide and single crystal silicon laminate film by RIE etching, and the vents are sealed by PECVD silicon dioxide under a certain pressure to achieve a cavity with a certain pressure inside.

3. A monolithic integrated multi-frequency CMUTs for wide range pressure measurement, characterized in that: At least one first CMUT unit, at least one second CMUT unit and at least one third CMUT unit are integrated on the chip; the second CMUT unit adopts the CMUT unit with adjustable cavity internal pressure as claimed in claim 1 or 2; By adjusting the diaphragm thickness, cavity height, diaphragm radius of the first, second and third CMUT units and the pressure control value in the cavity of the second CMUT unit, the first and second CMUT units are operated within the linear range of small deflection deformation, the third CMUT unit is operated 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, and the resonant frequencies do not overlap each other; The diaphragm thickness refers to the sum of the thickness of the silicon dioxide and single crystal silicon laminated films, and the diaphragm radius is equal to the cavity radius.

4. The monolithic integrated multi-frequency CMUTs for wide range pressure measurement according to claim 3, characterized in that: The diaphragm thickness of the first, second and third CMUT units is 5µm, the cavity height is 3µm, and the diaphragm radius is 120µm, 125µm and 200µm respectively.

5. The monolithic integrated multi-frequency CMUTs for wide range pressure measurement according to claim 4, characterized in that: The CMUT unit with adjustable internal cavity pressure has a cavity filled with gas at a pressure of 50 kPa.

6. The monolithic integrated multi-frequency CMUTs for wide range pressure measurement according to any one of claims 3 to 5, characterized in that: There are multiple first, second and third CMUT units, all arranged in a matrix, forming the first, second and third CMUTs respectively.

7. The monolithic integrated multi-frequency CMUTs for wide range pressure measurement according to any one of claims 3 to 5, characterized in that: The metal film is aluminum.

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