CN113714072B - High-sensitivity micro-pressure detection ring-shaped groove diaphragm structure capacitance type micro-mechanical ultrasonic transducer - Google Patents
High-sensitivity micro-pressure detection ring-shaped groove diaphragm structure capacitance type micro-mechanical ultrasonic transducer Download PDFInfo
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Abstract
The invention relates to the field of micro-pressure (0-10 kPa) detection, in particular to an MEMS or capacitive micro-mechanical ultrasonic transducer, and specifically relates to a high-sensitivity micro-transducerThe pressure detection ring-shaped groove diaphragm structure capacitance type micro-mechanical ultrasonic transducer comprises a metal upper electrode, a Si diaphragm and SiO 2 Support pillar, vacuum cavity and SiO 2 An insulating layer and a silicon substrate, wherein the lower edge of the Si diaphragm is provided with annular SiO 2 Support column, siO 2 A silicon substrate, a Si diaphragm and SiO are arranged below the supporting column 2 The cavity enclosed by the support column and the silicon substrate is a vacuum cavity, and SiO is arranged at the bottom of the vacuum cavity 2 And a metal upper electrode is sputtered on the insulating layer and the Si vibrating diaphragm, and a plurality of concentric annular grooves are etched on the upper surface of the Si vibrating diaphragm. The capacitive micro-machined ultrasonic transducer has the pressure detection performance of high sensitivity and excellent linearity.
Description
Technical Field
The invention relates to the field of micro-pressure (0-10 kPa) detection, in particular to an MEMS or capacitive micro-machined ultrasonic transducer, and specifically relates to a high-sensitivity micro-pressure detection ring-shaped groove diaphragm structure capacitive micro-machined ultrasonic transducer.
Background
The pressure is the most basic application parameter, and the precision measurement of the pressure is a technical problem which needs to be solved urgently in the fields of industrial production, scientific research, national defense science and technology, biological medical treatment and the like. Particularly, in a micro-pressure environment, efficiently and accurately acquiring weak pressure is always one of the key technologies and difficulties in pressure sensor design; in addition to the complex conditions of sudden pressure change and the like, the difficulty in designing a practical sensor with high sensitivity and low detection is quite high. Nevertheless, micro-pressure sensors are always the research focus of researchers, which mainly lies in the urgent application requirements of micro-pressure measurement in the fields of aerospace, industrial control and the like. For example: the method comprises the steps of carrying out height measurement and track correction of an aircraft through high altitude pressure, monitoring the pressure condition in the chemical reaction process in industrial production, detecting the pulsation air pressure of foreign objects in a high-speed rail running system, detecting the pressure difference before and after the carotid artery blood vessel operation and the like.
After a long time of development, sensors for pressure measurement are continuously developed and put into use, and more prominently, pressure sensors based on MEMS technology. Compared with other types of pressure sensors, the sensor has the remarkable advantages of miniaturization, high resonance frequency, high sensitivity, low noise, mass production and the like. MEMS pressure sensors can be broadly divided into: piezoresistive, capacitive and resonant types. The piezoresistive MEMS pressure sensor is prepared by utilizing the piezoresistive effect of materials. The sensor has simple process and good linearity, but the inherent temperature-sensitive characteristic has great influence on the measurement precision, and the necessary temperature compensation seriously limits the application range of the sensor. The capacitance MEMS pressure sensor indirectly obtains the pressure to be measured through the capacitance change between the upper and lower polar plates. The sensor has the advantages of low temperature drift, low power consumption, high sensitivity and the like, but the inherent nonlinear characteristic and small output capacitance have high requirements on detection signals. And the resonant MEMS pressure sensor indirectly realizes pressure measurement through the natural frequency change of the resonator during working. Compared with the former two, the resonant MEMS pressure sensor is mainly affected by mechanical characteristics of the device itself structural design, so that the measurement accuracy/sensitivity is high, the anti-interference performance is strong, and the sensor performance requirements in aerospace, industrial control and other fields are high, wherein the typical representative is a silicon micro-resonant MEMS pressure sensor.
As a representative of the silicon micro-resonance type MEMS sensor, a Capacitive Micromachined Ultrasonic Transducer (CMUT) has become an important component of the silicon micro-resonance type MEMS pressure sensor due to its excellent resonance characteristics. When the CMUT is designed as a pressure sensor, it can be used to measure static or slowly varying pressures. Different from a resonant pressure sensor with a second sensitive element such as a cantilever beam, the diaphragm of the CMUT can directly sense different pressures, so that the resolution is effectively improved; in addition, the CMUT itself has higher resonant frequency, quality factor and the like, and conditions are created for improving the pressure measurement sensitivity; furthermore, the robust structure of the CMUT, the good circuit matching, and the mass producibility have significant advantages in ensuring the reliability of pressure measurement in a complex environment.
At present, the structure of a typical Capacitive Micromachined Ultrasonic Transducer (CMUT) is shown in fig. 1, and comprises a metal upper electrode, a Si diaphragm, and SiO 2 Support pillar, vacuum cavity and SiO 2 An insulating layer and a silicon substrate (also a lower electrode), wherein the lower edge of the Si diaphragm is provided with annular SiO 2 Support post, siO 2 A silicon substrate, a Si diaphragm and SiO are arranged below the supporting column 2 The cavity enclosed by the support column and the heavily doped silicon substrate is a vacuum cavity, and SiO is arranged at the bottom of the vacuum cavity 2 And a metal upper electrode is sputtered above the insulating layer and the Si diaphragm, and the silicon substrate is a heavily doped silicon substrate. The traditional Si diaphragm of the capacitive micro-machined ultrasonic transducer is a flat diaphragm, and the diaphragm and the cylindrical cavity structure seriously limit the deformation of the diaphragm and inhibit the further optimization of the pressure measurement performance.
Disclosure of Invention
Common capacitive micro-machined ultrasonic transducers for micro-pressure (0-10 kPa) measurement are limited by a diaphragm structure of the transducers, and the pressure measurement sensitivity and linearity are low. In view of the above problem, an object of the present invention is to provide a CMUT micro-cell having a grooved membrane structure for micro-pressure detection to achieve high sensitivity and excellent linearity of pressure detection performance.
The invention is realized by adopting the following technical scheme: a high-sensitivity micro-pressure detection ring-shaped groove diaphragm structure capacitive micro-mechanical ultrasonic transducer comprises a metal upper electrode, a Si diaphragm and SiO 2 Support pillar, vacuum cavity and SiO 2 An insulating layer and a silicon substrate, wherein the lower edge of the Si diaphragm is provided with annular SiO 2 Support post, siO 2 A silicon substrate, a Si diaphragm and SiO are arranged below the supporting column 2 The cavity enclosed by the support column and the silicon substrate is a vacuum cavity, and SiO is arranged at the bottom of the vacuum cavity 2 Insulating layer, sputtering metal on top of Si diaphragmEtching a plurality of concentric annular grooves on the upper surface of the Si diaphragm; the circle center of the annular groove is located on the vertical central line of the Si diaphragm.
The basic working principle of the invention is as follows: the electrodes at two ends of the CMUT apply certain bias voltage and are placed in a micro-pressure environment, the displacement state of the slotted vibrating diaphragm is obviously changed under the combined action of electrostatic force and external pressure, the resonant frequency of the CMUT is further changed, and the measured pressure value can be obtained through the corresponding relation between the pressure change and the frequency change.
The high-sensitivity micro-pressure detection ring-shaped groove diaphragm structure capacitance type micro-mechanical ultrasonic transducer has 3 to 7 ring-shaped grooves; the annular grooves are the same in width and depth, and the distances between the adjacent annular grooves are the same.
In the capacitance type micro-mechanical ultrasonic transducer with the high-sensitivity micro-pressure detection annular groove diaphragm structure, the annular groove is located in the edge area of the Si diaphragm, and the shortest distance between the annular groove and the edge of the Si diaphragm is 15 micrometers. The annular groove is located in the edge area, and compared with the central area, the sensitivity improvement effect is more obvious. When the number of the annular grooves is 3-7, the sensitivity improvement effect is good.
In the capacitive micro-machined ultrasonic transducer with the high-sensitivity micro-pressure detection annular groove diaphragm structure, the width of each annular groove is 3.5 micrometers, the distance between every two adjacent annular grooves is 2 micrometers, and the depth of each annular groove is 3.5 micrometers.
The CMUT with the slotted membrane structure has the following advantages:
(1) After the CMUT is slotted, compared with the conventional CMUT microcell, the average thickness and bending stiffness of the diaphragm are reduced, and under the condition that the bias voltage and the external pressure are not changed, more obvious displacement change can be realized (as shown in fig. 3).
(2) After the CMUT micro-elements are grooved, compared with the conventional CMUT micro-elements, the small change of the diaphragm to the external pressure will generate more obvious displacement and response change of the resonant frequency, so that the ultra-high sensitivity pressure detection can be realized (as shown in fig. 4 and 5).
(3) After the CMUT micro-cell is subjected to slotting processing, compared with the conventional CMUT micro-cell, the linearity between the resonance frequency change and the pressure change is closer to 1, and the accuracy of the CMUT micro-cell in measuring the pressure can be effectively ensured (as shown in fig. 6).
(4) After the CMUT cells are subjected to slotting processing, compared with the traditional CMUT cells, the collapse voltage value is greatly reduced, so that the bias voltage value is closer to the collapse voltage value, and the sensitivity of pressure measurement can be effectively improved (as shown in fig. 7).
Drawings
Fig. 1 is a structural view of a conventional CMUT.
Fig. 2 is a diagram of the CMUT cell structure of the present invention.
Fig. 3 is a graph comparing the displacements of the two structures.
Fig. 4 is a graph comparing the frequency sensitivity and the pressure sensitivity of two structures, wherein the slopes of two oblique lines represent the pressure sensitivity of the two structures, respectively.
Fig. 5 is a graph comparing the sensitivity of the two structures.
Fig. 6 is a graph comparing the linearity of two structures.
FIG. 7 is a graph comparing collapse voltages for two structures.
Fig. 8 is a schematic diagram of the specification of the annular groove on the diaphragm.
FIG. 9 is a graph showing the relationship between the groove width of the circular groove and the sensitivity.
FIG. 10 is a graph showing the relationship between the number of annular grooves and the sensitivity.
In the figure: 1-metal upper electrode, 2-Si vibrating diaphragm and 3-SiO 2 Support column, 4-vacuum chamber, 5-SiO 2 Insulating layer, 6-silicon substrate, 7-annular trench.
Detailed Description
The high-sensitivity micro-pressure detection ring-shaped groove diaphragm structure capacitance type micro-mechanical ultrasonic transducer comprises a metal upper electrode 1, a Si diaphragm 2 and SiO 2 Support pillar 3, vacuum chamber 4, siO 2 An insulating layer 5 and a low resistivity silicon substrate (also a lower electrode); the specific preparation process flow comprises the following steps: firstly, removing surface insoluble pollutants from prepared SOI wafers and high-quality Si wafers after RCA cleaning; then, photoetching and inductively coupled plasma deep silicon etching process are adopted to etch high-quality Si crystalThe shape and the depth of the cavity are defined on the circle, and then a silicon dioxide insulating layer is formed through dry oxygen oxidation; secondly, after the SOI and the processed Si wafer are bonded, removing an oxygen burying layer and a substrate of the SOI by adopting a dry etching process and a wet etching process, and releasing top silicon to form a vibrating diaphragm; thirdly, defining a metal upper electrode by adopting photoetching and magnetron sputtering processes; finally, the required 5 annular grooves 7 are dry-etched on the edge of the diaphragm, as shown in fig. 8, and the width t of the groove 1 ＝……＝t 5 =3.5 μm, adjacent trench spacing g 1 All are the same, g 1 =2 μm, trench depth g 2 All are the same, g 2 ＝3.5μm；SiO 2 The support pillar width is typically 10 μm.
The CMUT micro-element except the Si vibrating diaphragm has the same structure as a typical CMUT micro-element, the annular groove of the vibrating diaphragm structure is positioned in the edge area of the vibrating diaphragm, and in order to ensure the stability of the mechanical structure of the final vibrating diaphragm, the outermost groove and the edge of the vibrating diaphragm are set with enough distance; the width, depth and spacing of all the annular grooves are all the same.
The invention is further described below with reference to material properties.
Table 1 shows the geometric parameters of a conventional CMUT, the radius of the diaphragm is 200 μm, and the data of the width of the ring-shaped groove and the support pillar in the previous embodiment show that the ring-shaped groove is located on the upper surface of the diaphragm near the edge.
Table 1 conventional CMUT geometric parameters
Table 2 shows the characteristic parameters of the materials used for CMUTs, where ρ represents the density, E is the Young's modulus, v is the Poisson's ratio, ε γ Is the relative dielectric constant;
TABLE 2 Material Property parameters
The properties of CMUTs are closely related to structure, material, load, etc., and they can affect the measurement performance of the pressure sensor by indirectly changing the diaphragm displacement. When the electrostatic force and the uniform pressure act together, the displacement at the radial position r of the diaphragm is:
wherein D is effective bending stiffness and P is pressure to be measured;
when the initial deflection is zero, the electrostatic force Q per unit area is as follows:
Q＝ε 0 V dc 2 /(2d 0 2 ) (2)
in the formula of 0 Relative dielectric constant of vacuum, V dc Is a bias voltage;
when the structure, material, load, etc. of the CMUT changes, the diaphragm displacement also changes. If the pressure P to be measured is constant, the bending rigidity of the diaphragm and the influence of the electrostatic force on the displacement of the diaphragm present opposite trends according to the formula (1). The increase of the electrostatic force can lead to the increase of the displacement of the diaphragm; whereas a reduction in the bending stiffness of the diaphragm increases its displacement. Meanwhile, the bending stiffness of the diaphragm is closely related to the structural design of the diaphragm, and the electrostatic force is influenced by the height of the closed cavity, so that the optimization of the structural design of the CMUT becomes an important means for improving the overall performance of the CMUT.
The radius, the thickness and the cavity height of the vibrating diaphragm with the novel structure are completely the same as those of the traditional structure, and the difference is that the traditional flat diaphragm structure is improved into a slotted diaphragm structure. The design performs slotting treatment on the edge of the traditional diaphragm to form a certain number of annular grooves. Therefore, the average thickness of the diaphragm is reduced, and according to the formula (3), the effective bending rigidity of the diaphragm is reduced, so that the deformability of the diaphragm is enhanced, and the pressure sensitivity, the linearity and other measurement performances of the CMUT are effectively improved.
D＝Eh 3 /[12(1-v 2 )] (3)
The innovations of the present invention are further described below in conjunction with simulation data.
TABLE 3 Single groove simulation data
As can be seen from the data in table 3, the pressure sensitivity and the fitting linearity of the CMUT are both improved with the increasing depth of the trench, and the linear relationship is always in the best state when the depth of the trench reaches 3.5 μm. Thus, to maximize pressure sensitivity, the larger groove depth should be selected, but should not be too large, mainly because continued increase in groove depth may result in excessive diaphragm displacement, increasing the risk of small deflection theoretical failure and diaphragm collapse.
TABLE 4 Multi-Trench Width simulation data
As can be seen from the data of table 4 in conjunction with fig. 9, the pressure sensitivity continued to increase with the increase in the trench width, but the rate of increase began to become slower at 3.5 μm. To ensure the mechanical stability of the diaphragm structure, the width of the groove is preferably 3.5 μm.
As can be seen from fig. 10, the pressure sensitivity is continuously improved as the number of the grooves is increased, but the curve gradually becomes gentle, i.e., the closer to the center of the diaphragm, the smaller the pressure sensitivity is increased and the closer to 0 is. Meanwhile, the 5-trench film structure is optimally designed in consideration of the complexity of the actual manufacturing process.
Claims (3)
1. A high-sensitivity micro-pressure detection ring-shaped groove diaphragm structure capacitance type micro-mechanical ultrasonic transducer comprises a metal upper electrode (1), a Si diaphragm (2) and SiO 2 Support pillar (3), vacuum cavity (4), siO 2 An insulating layer (5) and a silicon substrate (6), wherein the lower edge position of the Si diaphragm (2) is provided with annular SiO 2 Support column (3), siO 2 A silicon substrate (6), a Si diaphragm (2) and an S are arranged below the support column (3)iO 2 A cavity enclosed by the support column (3) and the silicon substrate (6) is a vacuum cavity (4), and SiO is arranged at the bottom of the vacuum cavity (4) 2 Insulating layer (5), si vibrating diaphragm top sputtering metal upper electrode (1), its characterized in that: a plurality of concentric annular grooves (7) are etched on the upper surface of the Si diaphragm (2); the circle center of the annular groove (7) is positioned on the vertical central line of the Si vibrating diaphragm (2); the number of the annular grooves (7) is 3~7; the width and the depth of the annular grooves (7) are the same, and the distance between the adjacent annular grooves (7) is the same; the annular groove (7) is positioned in the edge area of the Si diaphragm (2), and the shortest distance between the outer edge of the annular groove (7) positioned at the outermost periphery and the edge of the Si diaphragm (2) is 15 microns; the widths of the annular grooves (7) are all 3.5 micrometers, the distance between every two adjacent annular grooves (7) is 2 micrometers, and the depth of each annular groove (7) is 3.5 micrometers.
2. The high-sensitivity micro-pressure detection annular groove diaphragm structure capacitive micro-machined ultrasonic transducer of claim 1, characterized in that: the number of the annular grooves (7) is 5.
3. The high-sensitivity micro-pressure detection annular groove diaphragm structure capacitive micro-machined ultrasonic transducer of claim 1, characterized in that: the silicon substrate (6) is a low-resistivity silicon substrate.
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| US20060004289A1 (en) * | 2004-06-30 | 2006-01-05 | Wei-Cheng Tian | High sensitivity capacitive micromachined ultrasound transducer |
| US7615834B2 (en) * | 2006-02-28 | 2009-11-10 | The Board Of Trustees Of The Leland Stanford Junior University | Capacitive micromachined ultrasonic transducer(CMUT) with varying thickness membrane |
| JP2009182838A (en) * | 2008-01-31 | 2009-08-13 | Kyoto Univ | Elastic wave transducer, elastic wave transducer array, ultrasonic probe, and ultrasonic imaging apparatus |
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| CN109092649B (en) * | 2018-09-05 | 2020-05-22 | 西安交通大学 | Electrostatic-piezoelectric hybrid driving transceiving integrated CMUT and using method and preparation method thereof |
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