CN111579426B - High-quality factor piezoelectric cantilever beam density sensor chip and working method and preparation method thereof - Google Patents
High-quality factor piezoelectric cantilever beam density sensor chip and working method and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a high-quality factor piezoelectric cantilever beam density sensor chip and a working method and a preparation method thereof. The MEMS technology is adopted to enable the silicon micro-resonance cantilever beam structure to be covered with a low-stress aluminum nitride piezoelectric film, the double piezoelectric electrodes are used for introducing alternating voltage with certain frequency and generating piezoelectric driving force based on inverse piezoelectric effect, four sensitive resistance strips on the four piezoresistive beams are connected through metal leads on the piezoresistive connecting beams to form a Wheatstone full bridge, the Wheatstone full bridge is used for detecting resonance stress and converting the resonance stress into voltage signals to be output through arranging the Wheatstone bridge, an out-of-plane vibration mode of the cantilever beam can be obtained through a piezoelectric excitation mode, the density sensor chip has high sensitivity and high quality factors in fluid, the application range of fluid density measurement can be remarkably improved, and the measurement precision and the sensitivity are high.
Description
Technical Field
The invention relates to the field of MEMS (Micro electro mechanical Systems, Micro mechanical electronic Systems) sensors, in particular to a high-quality factor piezoelectric cantilever beam density sensor chip and a working method and a preparation method thereof.
Background
The resonant density sensor based on the MEMS technology detects the fluid density characteristic, and has the advantages of small volume, easy operation, high sensitivity and the like compared with the traditional densimeter by relying on the change of the resonant frequency of a resonator caused by the change of the attached additional mass of fluid molecules. However, the viscosity of the fluid has a considerable effect on the quality factor of the density resonator, the vibration stability of the resonator, and the applicability and measurement accuracy of the density measurement of the medium and high viscosity fluid, and along with the high density detection requirements of various industries on the low, medium and high viscosity fluids, the MEMS density sensor still faces significant problems of narrow application range, low sensitivity, poor convenience and the like.
The quality factor and the density measurement sensitivity are important indexes of the MEMS resonant density sensor, Newton, non-Newton liquid and low-viscosity liquid all have obvious influence on the quality factor of a resonator of the MEMS resonant density sensor, and when the density resonator based on out-of-plane vibration interacts with fluid molecules, a larger length-width surface is taken as a leading factor, so that the quality factor is influenced severely by fluid damping due to larger squeeze film fluid damping while the density sensitivity is increased; on the contrary, although the density resonance based on in-plane vibration can remarkably improve the fluid-resistant damping characteristic due to the damping effect of the slip film, the vibration form of the density resonance enables the effect of the density resonance and fluid molecules to be only the width-thickness surface of the resonator, thereby greatly reducing the frequency change caused by the change of the fluid density and reducing the sensitivity of density measurement. Therefore, the quality factor of the MEMS density resonance sensor is improved, and the MEMS density resonance sensor has good density measurement sensitivity, and is an important development direction of the sensors at present.
Disclosure of Invention
In order to solve the problems in the prior art, the invention aims to provide a high-quality-factor piezoelectric cantilever beam density sensor chip, a working method and a preparation method thereof, so as to simultaneously improve the quality factor and the density measurement sensitivity of the sensor chip, thereby improving the density measurement precision, the sensitivity and the application range, and simultaneously considering the working convenience and the reliability of the sensor.
In order to achieve the purpose, the invention adopts the following technical scheme:
a high-quality factor piezoelectric cantilever beam density sensor chip comprises a silicon substrate and a silicon micro-cantilever beam resonator, wherein the silicon micro-cantilever beam resonator comprises a micro-cantilever beam suspension structure, a clamped beam structure, a piezoresistive beam structure and a piezoresistive connecting beam structure; the silicon substrate is provided with a cavity, the micro-cantilever beam suspension structure is arranged in the cavity, and the micro-cantilever beam suspension structure is connected with the silicon substrate through the clamped beam structure;
the piezoresistive connecting beam comprises a first piezoresistive connecting beam and a second piezoresistive connecting beam, and the first piezoresistive connecting beam and the second piezoresistive connecting beam are symmetrically arranged on two sides of the clamped beam structure along the width direction;
the piezoresistive beam structure comprises a first piezoresistive beam, a second piezoresistive beam, a third piezoresistive beam and a fourth piezoresistive beam; one end of the first piezoresistive beam is connected with the micro-cantilever beam suspension structure, and the other end of the first piezoresistive beam is connected with one side of the first piezoresistive connecting beam; one end of the second piezoresistive beam is connected with the other side of the first piezoresistive connecting beam, and the other end of the second piezoresistive beam is connected with the silicon substrate; one end of the third piezoresistive beam is connected with one side of the second piezoresistive connecting beam, and the other end of the third piezoresistive beam is connected with the silicon substrate; one end of the fourth piezoresistive beam is connected with the micro-cantilever beam suspension structure, and the other end of the fourth piezoresistive beam is connected with the other side of the second piezoresistive connecting beam; the first piezoresistive beam and the fourth piezoresistive beam are symmetrically arranged relative to the clamped beam structure, the second piezoresistive beam and the third piezoresistive beam are symmetrically arranged relative to the clamped beam structure, the first piezoresistive beam and the second piezoresistive beam are asymmetrically arranged relative to the first piezoresistive connecting beam, and the third piezoresistive beam and the fourth piezoresistive beam are asymmetrically arranged relative to the second piezoresistive connecting beam;
the two sides of the clamped beam structure in the width direction are respectively provided with a first piezoelectric electrode and a second piezoelectric electrode, and the first piezoelectric electrode and the second piezoelectric electrode both comprise a top electrode and a bottom electrode;
the first piezoresistive beam, the second piezoresistive beam, the third piezoresistive beam, the fourth piezoresistive beam, the first piezoelectric electrode and the second piezoelectric electrode are all connected with a bonding pad through metal leads, and the first piezoresistive beam, the second piezoresistive beam, the third piezoresistive beam and the fourth piezoresistive beam are electrically connected to form a Wheatstone full bridge.
Preferably, the spacing between the first piezoresistive beam and the fourth piezoresistive beam is greater than the spacing between the second piezoresistive beam and the third piezoresistive beam.
Preferably, the first, second, third and fourth piezoresistive beams are all the same in length, width and thickness; the first and second piezoresistive engagement beams are all the same in length, width and thickness.
Preferably, the top electrode and the bottom electrode of the first piezoelectric electrode are the same in shape and size as the top electrode and the bottom electrode of the second piezoelectric electrode.
Preferably, the invention provides a specific size of the high-quality factor piezoelectric cantilever density sensor chip, and the size of the micro-cantilever suspension structure is as follows: length × width (1100 ± 5) × (1400 ± 5) μm2The thickness is 30 +/-10 mu m; the size of the clamped beam structure is as follows: length × width (280 ± 5) × (270 ± 5) × μm2The thickness is 30 +/-10 mu m; the dimensions of the first, second, third and fourth piezoresistive beams are: length × width (125 ± 3) × (8 ± 3) × μm2The thickness is 30 +/-10 mu m; the dimensions of the first piezoresistive connection beam and the second piezoresistive connection beam are as follows: length × width (30 ± 5) × (320 ± 5) × μm2The thickness is 30 +/-10 mu m; the distance between the first piezoresistive beam and the clamped beam structure and the distance between the fourth piezoresistive beam and the clamped beam structure are as follows: 160 +/-5 mu m; the distance between the second piezoresistive beam and the clamped beam structure and the distance between the third piezoresistive beam and the clamped beam structure are as follows: 260 +/-5 mu m; the width of the metal lead wire is: 20 + -10 μm.
The working method of the high-quality factor piezoelectric cantilever beam density sensor chip comprises the following steps of:
when the density of a measured fluid is measured, in-phase alternating sinusoidal voltage is simultaneously introduced to the first piezoelectric electrode and the second piezoelectric electrode, a top electrode electrification and bottom electrode grounding mode of the double piezoelectric electrodes is formed, according to the inverse piezoelectric effect, the clamped beam structure drives the micro-cantilever beam suspension structure to vibrate under the action of the double piezoelectric driving force, when the alternating voltage frequency is close to the inherent frequency of out-of-plane resonance of the silicon micro-cantilever beam resonator, an out-of-plane resonance mode is generated, the first piezoresistive beam, the second piezoresistive beam, the third piezoresistive beam and the fourth piezoresistive beam are driven to vibrate, and alternating voltage detection signals are output through a Wheatstone full bridge.
The preparation method of the high-quality factor piezoelectric cantilever beam density sensor chip comprises the following steps of:
step 1: oxidizing the N-type (100) SOI silicon wafer on both sides, and generating thermal oxidation silicon dioxide layers on the front side and the back side;
step 2: etching the front surface of the SOI silicon wafer after the step 1, etching away thermal oxidation silicon dioxide in a region corresponding to the piezoresistive beam structure, using thermal oxidation silicon dioxide layers in other regions as masks, lightly doping boron ions on the exposed device layer, and using the lightly doped boron ion region as a sensitive piezoresistive strip on the piezoresistive beam structure;
and step 3: etching the front surface of the SOI silicon wafer after the step 2, etching thermal oxidation silicon dioxide in a corresponding area at the end part of the piezoresistive beam, and carrying out boron ion heavy doping on the exposed device layer to form an ohmic contact area with low resistance;
and 4, step 4: performing film deposition on the front side and the back side of the SOI silicon wafer subjected to the step 3 to sequentially prepare a silicon nitride layer and a silicon dioxide layer;
and 5: depositing a metal film on the front surface of the SOI silicon wafer after the step 4, wherein the deposited metal film is used as a substrate of the piezoelectric aluminum nitride film and also used as a bottom electrode of the piezoelectric electrode;
step 6: performing piezoelectric AlN film sputtering on the front surface of the SOI silicon wafer after the step 5 to obtain an AlN film;
and 7: etching a lead hole on the front surface of the SOI silicon wafer subjected to the step 6 by a dry method, then sputtering a metal electrode layer, and forming a piezoelectric electrode, a metal lead electrically connected with the sensitive piezoresistance strip and a bonding pad by utilizing a stripping process;
and 8: and (3) carrying out dry etching on the SOI silicon chip after the step (7), and sequentially carrying out dry etching on the front surface area and the back cavity area, so that the release of the silicon micro-cantilever resonator is realized, and the high-quality factor piezoelectric cantilever density sensor chip is prepared.
Preferably, in step 4, a silicon nitride layer and a silicon dioxide layer are sequentially prepared by using a chemical vapor deposition technique.
Preferably, in step 5, the metal film is a molybdenum film.
Preferably, in step 5, a metal film is deposited by utilizing a magnetron sputtering technology; and 6, preparing the AlN thin film by using a magnetron sputtering technology.
The invention has the following beneficial effects:
the high-quality factor piezoelectric cantilever beam density sensor chip takes a silicon micro-resonance cantilever beam structure as a resonance device, the large surface area of the chip increases the action area with fluid molecules, the density measurement sensitivity is improved, and the design requirements of a density sensor on low cost and high reliability are met; the resonator can excite out-of-plane vibration modes through piezoelectricity and has the characteristics of out-of-plane and in-plane vibration, so that the damping effect of the fluid can be reduced, the action area of the fluid can be considered, and the quality factor and the density measurement sensitivity of the sensor can be improved; the high-quality factor piezoelectric cantilever beam density sensor chip provided by the invention adopts piezoelectric excitation and piezoresistive detection working modes, has the advantages of easiness in operation, good reliability and the like, provides convenience for the miniaturization and integrated packaging of the density sensor chip, and improves the use convenience of the density sensor. According to the invention, the first piezoresistive beam and the second piezoresistive beam are asymmetrically arranged relative to the first piezoresistive connecting beam, and the third piezoresistive beam and the fourth piezoresistive beam are asymmetrically arranged relative to the second piezoresistive connecting beam, namely the piezoresistive beams are asymmetrically arranged along the width direction of the piezoresistive connecting beams, so that the coupling of tensile stress and compressive stress caused by out-of-plane bending on a single piezoresistive beam can be avoided.
Furthermore, the distance between the first piezoresistive beam and the fourth piezoresistive beam is larger than the distance between the second piezoresistive beam and the third piezoresistive beam, the effective resonance tension/compression stress on the piezoresistive beams can be improved by the arrangement mode, and the piezoresistive vibration pickup signal output, the density detection precision and the density detection sensitivity of the density detection are enhanced.
The preparation method of the high-quality factor piezoelectric cantilever beam density sensor chip is simple and easy to implement, and the prepared sensor chip can effectively solve the problems of narrow density measurement range, low sensitivity, poor convenience and the like.
Drawings
FIG. 1 is a schematic diagram of the overall structure of a high Q-factor piezoelectric cantilever density sensor chip according to the present invention.
Fig. 2 is an enlarged schematic view of a portion a in fig. 1.
FIG. 3 is a simulation diagram of the out-of-plane vibration modes of the silicon micro-cantilever resonator of the present invention.
FIG. 4 is a graph of the frequency domain characteristics of the sensor chip of the present invention in ethanol.
FIG. 5 is a graph of the frequency of the sensor chip of the present invention in five different fluids.
FIG. 6 is a flow chart of a process for manufacturing a high quality factor piezoelectric cantilever density sensor chip according to the present invention.
In the figure, 1-micro cantilever beam suspension structure, 2-1-first piezoresistive beam, 2-2-second piezoresistive beam, 2-3-third piezoresistive beam, 2-4-fourth piezoresistive beam, 3-metal lead, 4-bonding pad, 5-silicon substrate, 5-1-cavity, 6-clamped beam structure, 7-first piezoelectric electrode, 8-second piezoelectric electrode, 9-1-first piezoresistive connecting beam, 9-2-second piezoresistive connecting beam, 10-thermal oxygen silicon dioxide layer, 11-1-device layer, 11-2-buried oxygen layer, 11-3-substrate layer, 12-sensitive piezoresistive strip, 13-ohm contact area, 14-1-silicon dioxide layer, 14-2-silicon nitride layer, 15-molybdenum film, 16-AlN film, 17-piezoelectric electrode.
Detailed Description
Preferred embodiments of the present invention will now be described in further detail, by way of example, with reference to the accompanying drawings.
As shown in fig. 1 and 2, the high quality factor piezoelectric cantilever density sensor chip of the present invention includes a silicon substrate 5 and a silicon micro-cantilever resonator; the silicon micro-cantilever resonator comprises a micro-cantilever suspension structure 1, a clamped beam structure 6, a piezoresistive beam structure and a piezoresistive connecting beam structure; the piezoresistive connecting beams comprise a first piezoresistive connecting beam 9-1 and a second piezoresistive connecting beam 9-2, and the first piezoresistive connecting beam 9-1 and the second piezoresistive connecting beam 9-2 are symmetrically arranged on two sides of the fixed beam structure 6 along the width direction; the piezoresistive beam structure comprises a first piezoresistive beam 2-1, a second piezoresistive beam 2-2, a third piezoresistive beam 2-3 and a fourth piezoresistive beam 2-4; one end of the first piezoresistive beam 2-1 is connected with the micro cantilever beam suspension structure 1, and the other end of the first piezoresistive beam 2-1 is connected with one side of the first piezoresistive connecting beam 9-1; one end of the second piezoresistive beam 2-2 is connected with the other side of the first piezoresistive connecting beam 9-1, and the other end of the second piezoresistive beam 2-2 is connected with the silicon substrate 5; one end of the third piezoresistive beam 2-3 is connected with the other side of the second piezoresistive connecting beam 9-2, and the other end of the third piezoresistive beam 2-3 is connected with the silicon substrate 5; one end of a fourth piezoresistive beam 2-4 is connected with the micro-cantilever beam suspension structure 1, and the other end of the fourth piezoresistive beam 2-4 is connected with one side of a second piezoresistive connecting beam 9-2; the first piezoresistive beam 2-1 and the fourth piezoresistive beam 2-4 are symmetrically arranged with respect to the clamped beam structure 6, the second piezoresistive beam 2-2 and the third piezoresistive beam 2-3 are symmetrically arranged with respect to the clamped beam structure 6, the first piezoresistive beam 2-1 and the second piezoresistive beam 2-2 are asymmetrically arranged with respect to the first piezoresistive joined beam 9-1, and the third piezoresistive beam 2-3 and the fourth piezoresistive beam 2-4 are asymmetrically arranged with respect to the second piezoresistive joined beam 9-2. The arrangement form of the first piezoresistive beam 2-1, the second piezoresistive beam 2-2, the third piezoresistive beam 2-3 and the fourth piezoresistive beam 2-4 can detect the resonance stress of the silicon micro-cantilever resonator in the out-of-plane vibration to the maximum extent, the coupling of the tension stress and the compression stress on the piezoresistive beams caused by the out-of-plane bending can be avoided, the distance between the piezoresistive beam connected with the suspension structure of the micro-cantilever and the clamped beam is larger than the distance between the piezoresistive beam connected with the silicon substrate and the clamped beam, the same resonance stress is ensured to be formed between the symmetrical piezoresistive beams, the asymmetrical piezoresistive beams have the resonance stress close to the maximum extent, the effective resonance tension/compression stress on the piezoresistive beams is improved, and the output and reliability of the piezoresistive vibration pickup signal detected by density are enhanced;
the piezoelectric electrodes comprise a first piezoelectric electrode 7 and a second piezoelectric electrode 8, the first piezoelectric electrode 7 and the second piezoelectric electrode 8 both comprise a top electrode and a bottom electrode, the bottom electrode is arranged below the piezoelectric aluminum nitride film, so the structure diagram only shows the top electrode, the bottom electrode and the top electrode are completely the same in size and shape, the bottom electrode and the top electrode are symmetrically arranged along the thickness direction relative to the piezoelectric aluminum nitride film, and the bottom electrode is used for grounding; the first piezoelectric electrode 7 and the second piezoelectric electrode 8 are symmetrically arranged along the width direction of the clamped beam structure 6.
The micro-cantilever beam suspension structure 1 is used as a sensitive structure of fluid density property, an out-of-plane resonance mode can be excited through an inverse piezoelectric effect, and the resonance form of the cantilever beam is similar to a wave shape.
The sensor chip adopts an out-of-plane vibration mode as a working mode to measure the density of the fluid, the resonance displacement amplitude of the suspension structure of the micro-cantilever beam under the mode presents nonlinear distribution along the length direction, so that the vibration direction can be divided into components along the thickness direction and the length direction of the sensor chip, and the resonance displacement in the thickness direction is taken as a leading factor, so that the mode has the characteristics of out-of-plane and in-plane vibration at the same time, and the out-of-plane vibration mode is taken as the leading factor.
The metal lead wire can be divided into a part connected with the piezoresistance and a part connected with the piezoelectric electrode; wherein, part of the metal lead wire connected with the piezoresistors is used for connecting the sensitive piezoresistors to form a Wheatstone full bridge, and constant current is introduced to generate output alternating voltage based on the piezoresistance effect; and part of metal leads connected with the piezoelectric electrodes are respectively connected with the first piezoelectric electrode and the second piezoelectric electrode and used for providing alternating voltage for the AlN thin film and providing piezoelectric driving force for the sensor chip through inverse piezoelectric effect. The tail end of a clamped beam structure of the silicon micro-cantilever resonator is fixedly connected with the silicon substrate.
The working modes of piezoelectric excitation and piezoresistive detection are adopted. The metal lead wire that links to each other with two piezoelectric electrodes lets in homophase alternating sinusoidal voltage simultaneously, constitute the top electrode circular telegram of two piezoelectric electrodes, bottom electrode ground connection mode, according to the reverse piezoelectric effect, gu the beam structure 6 can drive little cantilever beam unsettled structure 1 and produce the vibration under the effect of two piezoelectric drive, when alternating voltage frequency is close to the off-plane resonance natural frequency of silicon little cantilever beam syntonizer, just can produce the off-plane resonance mode, the vibration of pressure drag roof beam makes the sensitive pressure strip on it receive the sensor vibration, and output alternating voltage detection signal based on pressure drag effect and Wheatstone full-bridge. The piezoresistive roof beam adopts along the asymmetric arrangement that the piezoresistive links up roof beam width direction, avoids on the single piezoresistive roof beam because the pulling pressure stress coupling that the off-plate bending brought, arranges the piezoresistive roof beam in the cantilever end that is close to the piezoresistive links up the roof beam simultaneously for improve the effective resonance on the piezoresistive roof beam and pull/press stress, the output that the signal of vibration was picked up to the reinforcing piezoresistive. The working modes of piezoelectric excitation and piezoresistive detection are adopted, so that the vibration driving force and the vibration stability of the resonator in fluid are effectively improved, and effective and reliable vibration signal detection is performed.
As an example, the dimensions of the micro-cantilever suspension structure 1 are: length × width 1100 × 1400 μm2The size of the clamped beam structure 6 is as follows: length x width 280 x 270 um2The dimensions of the piezoresistive beam structure are as follows: length x width 125 x 8 μm2The size of the piezoresistive connecting beam structure is as follows: length x width of 30 x 320 um2The distance between the first piezoresistive beam 2-1 and the clamped beam structure 6 and the distance between the fourth piezoresistive beam 2-4 and the clamped beam structure 6 are as follows: 260 μm, the distance between the second piezoresistive beam 2-2 and the clamped beam structure 6 and the distance between the third piezoresistive beam 2-3 and the clamped beam structure 6 are: 160 μm, metal lead width: 20 μm and the thickness of the silicon micro-cantilever resonator is 30 μm.
As shown in fig. 3, the silicon micro-cantilever resonator having the above dimensions is subjected to fluid-solid coupling simulation analysis by Comsol Multiphysics simulation software, and the resonator is an out-of-plane resonance mode having components in the thickness z-axis direction and the length x-axis direction and dominated by the resonance displacement in the thickness z-axis direction, so that the wave mode has morphological characteristics of out-of-plane and in-plane vibrations at the same time and dominated by the out-of-plane vibration mode. The simulated frequency domain curve is shown in FIG. 4, and ethanol (density of 789.4 kg/m) is selected3Viscosity of 1.2cP) is simulated fluid, the resonance frequency of the out-of-plane vibration mode is 71kHz, the quality factor calculated by using a half-power method can reach 139, and the quality factor of the sensor chip can be obviously increased by the vibration mode. As shown in fig. 5, five common alkanes and alcohols are selected for fluid-solid coupling simulation analysis to obtain a resonant frequency variation curve in different fluids, and the slope of the curve is the density measurement of the sensor chipSensitivity of 34 Hz/(kg. m)3). Therefore, the piezoelectric cantilever beam density sensor chip has good density measurement sensitivity while meeting high quality factors.
As shown in fig. 6, the method for manufacturing a high quality factor piezoelectric cantilever density sensor chip according to the present invention comprises the following steps:
step 1: using an N-type (100) SOI silicon chip, wherein the thicknesses of a device layer 11-1, an oxygen burying layer 11-2 and a substrate layer 11-3 of the SOI silicon chip are respectively 30 microns, 1 micron and 300 microns, and oxidizing the SOI silicon chip on two sides to generate thermal oxidation silicon dioxide layers 10 on the front side and the back side of the silicon chip;
step 2: etching the front surface of the SOI silicon wafer after the step 1, etching thermal silica in the piezoresistive beam region, using the thermal silica layer in the rest region as a mask, performing boron ion light doping on the exposed device layer, using the boron ion light doped region as a sensitive piezoresistive strip 12 on the piezoresistive beam, and making the piezoresistive value at 25 ℃ be (4400 +/-50) omega;
and step 3: etching the front surface of the SOI silicon wafer after the step 2, etching away the thermal oxidation silicon dioxide in the corresponding region, and carrying out boron ion heavy doping on the exposed device layer to form an ohmic contact region 13 with low resistance;
and 4, step 4: depositing silicon dioxide layers and silicon nitride layers on the front side and the back side of the SOI silicon wafer after the step 3, and preparing a silicon dioxide layer 14-1 with the deposition thickness of 100-200 nm and a silicon nitride layer 14-2 with the deposition thickness of 200-400 nm by utilizing a chemical vapor deposition (PECVD) technology;
and 5: performing molybdenum film deposition on the front surface of the SOI silicon wafer after the step 4, and depositing a molybdenum film 15 by utilizing a magnetron sputtering technology, wherein the thickness of the molybdenum film is 100-200 nm, and the molybdenum film is used as a substrate of a piezoelectric aluminum nitride film and a piezoelectric bottom electrode;
step 6: performing piezoelectric AlN film sputtering on the front surface of the SOI silicon wafer after the step 5, and preparing the AlN film 16 with the thickness of (1 +/-0.5) mu m by utilizing a magnetron sputtering technology;
and 7: etching a lead hole on the front surface of the SOI silicon wafer after the step 6 by a dry method, then sputtering a metal electrode layer, and forming a piezoelectric electrode 17 and a metal lead 3 and a bonding pad 4 which are electrically connected with the sensitive piezoresistive strip 12 by a stripping process;
and 8: and (4) carrying out dry etching on the SOI silicon chip after the step (7), and sequentially carrying out dry etching on the front surface area and the back cavity area, so that the release of the silicon micro-cantilever resonator is realized, and the high-quality factor piezoelectric cantilever density sensor chip is prepared.
The main technical indexes of the high-quality factor piezoelectric cantilever beam density sensor chip prepared by the invention are as follows:
1. resonance frequency: 60 kHz-80 kHz
2. The applicable range of the density is as follows: 100kg/m3～1500kg/m3
3. The applicable range of the viscosity is as follows: 0.1 to 50 mPas;
4. and (3) measuring precision: better than +/-0.3% FS;
5. working temperature: -20 ℃ to 80 ℃;
the above description is only one embodiment of the present invention, and not all or only one embodiment, and any equivalent alterations to the technical solutions of the present invention, which are made by those skilled in the art through reading the present specification, are covered by the claims of the present invention.
Claims (9)
1. A high-quality factor piezoelectric cantilever beam density sensor chip is characterized by comprising a silicon substrate (5) and a silicon micro-cantilever beam resonator, wherein the silicon micro-cantilever beam resonator comprises a micro-cantilever beam suspension structure (1), a clamped beam structure (6), a piezoresistive beam structure and a piezoresistive connecting beam structure; a cavity (5-1) is arranged on the silicon substrate (5), the micro-cantilever beam suspension structure (1) is arranged in the cavity (5-1), and the micro-cantilever beam suspension structure (1) is connected with the silicon substrate (5) through a support beam fixing structure (6);
the piezoresistive connecting beams comprise a first piezoresistive connecting beam (9-1) and a second piezoresistive connecting beam (9-2), and the first piezoresistive connecting beam (9-1) and the second piezoresistive connecting beam (9-2) are symmetrically arranged on two sides of the fixed supporting beam structure (6) along the width direction;
the piezoresistive beam structure comprises a first piezoresistive beam (2-1), a second piezoresistive beam (2-2), a third piezoresistive beam (2-3) and a fourth piezoresistive beam (2-4); one end of the first piezoresistive beam (2-1) is connected with the micro-cantilever beam suspension structure (1), and the other end of the first piezoresistive beam (2-1) is connected with one side of the first piezoresistive connecting beam (9-1); one end of the second piezoresistive beam (2-2) is connected with the other side of the first piezoresistive connecting beam (9-1), and the other end of the second piezoresistive beam (2-2) is connected with the silicon substrate (5); one end of the third piezoresistive beam (2-3) is connected with one side of the second piezoresistive connecting beam (9-2), and the other end of the third piezoresistive beam (2-3) is connected with the silicon substrate (5); one end of a fourth piezoresistive beam (2-4) is connected with the micro-cantilever beam suspension structure (1), and the other end of the fourth piezoresistive beam (2-4) is connected with the other side of the second piezoresistive linking beam (9-2); the first piezoresistive beam (2-1) and the fourth piezoresistive beam (2-4) are symmetrically arranged relative to the clamped beam structure (6), the second piezoresistive beam (2-2) and the third piezoresistive beam (2-3) are symmetrically arranged relative to the clamped beam structure (6), the first piezoresistive beam (2-1) and the second piezoresistive beam (2-2) are asymmetrically arranged relative to the first piezoresistive clamped beam (9-1), and the third piezoresistive beam (2-3) and the fourth piezoresistive beam (2-4) are asymmetrically arranged relative to the second piezoresistive clamped beam (9-2);
the two sides of the clamped beam structure (6) in the width direction are respectively provided with a first piezoelectric electrode (7) and a second piezoelectric electrode (8), and the first piezoelectric electrode (7) and the second piezoelectric electrode (8) both comprise a top electrode and a bottom electrode;
the first piezoresistive beam (2-1), the second piezoresistive beam (2-2), the third piezoresistive beam (2-3), the fourth piezoresistive beam (2-4), the first piezoelectric electrode (7) and the second piezoelectric electrode (8) are all connected with a bonding pad through metal leads, and the first piezoresistive beam (2-1), the second piezoresistive beam (2-2), the third piezoresistive beam (2-3) and the fourth piezoresistive beam (2-4) are electrically connected to form a Wheatstone full bridge.
2. A high quality factor piezoelectric cantilever beam density sensor chip according to claim 1, wherein the spacing between the first piezoresistive beam (2-1) and the fourth piezoresistive beam (2-4) is larger than the spacing between the second piezoresistive beam (2-2) and the third piezoresistive beam (2-3).
3. The high quality factor piezoelectric cantilever beam density sensor chip of claim 1, wherein the first piezoresistive beam (2-1), the second piezoresistive beam (2-2), the third piezoresistive beam (2-3) and the fourth piezoresistive beam (2-4) have the same length, width and thickness; the first piezoresistive connecting beam (9-1) and the second piezoresistive connecting beam (9-2) are the same in length, width and thickness.
4. A high quality factor piezoelectric cantilever density sensor chip according to claim 1, wherein the top and bottom electrodes of the first piezoelectric electrode (7) and the top and bottom electrodes of the second piezoelectric electrode (8) are the same in shape and size.
5. The method of operating a high quality factor piezoelectric cantilever density sensor chip according to any one of claims 1 to 4, comprising the steps of:
when the density of a measured fluid is measured, in-phase alternating sinusoidal voltage is simultaneously introduced to a first piezoelectric electrode (7) and a second piezoelectric electrode (8), a top electrode power-on mode and a bottom electrode grounding mode of double piezoelectric electrodes are formed, according to the inverse piezoelectric effect, a clamped beam structure (6) drives a micro-cantilever beam suspension structure (1) to vibrate under the action of double piezoelectric driving force, when the frequency of alternating voltage is close to the inherent frequency of out-of-plane resonance of a silicon micro-cantilever beam resonator, an out-of-plane resonance mode is generated, a first piezoresistive beam (2-1), a second piezoresistive beam (2-2), a third piezoresistive beam (2-3) and a fourth piezoresistive beam (2-4) are driven to vibrate, and alternating voltage detection signals are output through a Wheatstone full bridge.
6. The method for preparing a high quality factor piezoelectric cantilever density sensor chip according to any one of claims 1 to 4, comprising the steps of:
step 1: double-sided oxidation of an N-type (100) SOI wafer to generate a thermal oxidation silicon dioxide layer (10) on both sides;
step 2: etching the front surface of the SOI silicon wafer after the step 1, etching hot oxygen silicon dioxide in a corresponding area of the piezoresistive beam structure, using a hot oxygen silicon dioxide layer in the rest area as a mask, lightly doping boron ions on the exposed device layer, and using the lightly doped boron ion area as a sensitive piezoresistive strip (12) on the piezoresistive beam structure;
and step 3: etching the front surface of the SOI silicon wafer after the step 2, etching thermal oxidation silicon dioxide in a corresponding area at the end part of the piezoresistive beam, and carrying out boron ion heavy doping on the exposed device layer to form an ohmic contact area (13) with low resistance;
and 4, step 4: performing film deposition on the front surface and the back surface of the SOI silicon wafer subjected to the step 3 to sequentially prepare a silicon nitride layer (14-2) and a silicon dioxide layer (14-1);
and 5: depositing a metal film on the front surface of the SOI silicon wafer after the step 4, wherein the deposited metal film is used as a substrate of the piezoelectric aluminum nitride film and also used as a bottom electrode of the piezoelectric electrode;
step 6: performing piezoelectric AlN film sputtering on the front surface of the SOI silicon wafer after the step 5 to obtain an AlN film (16);
and 7: etching a lead hole on the front surface of the SOI silicon wafer subjected to the step 6 by a dry method, then sputtering a metal electrode layer, and forming a piezoelectric electrode, a metal lead electrically connected with the sensitive piezoresistance strip and a bonding pad by utilizing a stripping process;
and 8: and (3) carrying out dry etching on the SOI silicon chip after the step (7), and sequentially carrying out dry etching on the front surface area and the back cavity area, so that the release of the silicon micro-cantilever resonator is realized, and the high-quality factor piezoelectric cantilever density sensor chip is prepared.
7. The method of claim 6, wherein in step 4, the silicon nitride layer (14-2) and the silicon dioxide layer (14-1) are sequentially formed by a chemical vapor deposition technique.
8. The method according to claim 6, wherein the metal thin film in step 5 is a molybdenum thin film.
9. The method according to claim 6, wherein in step 5, a metal thin film is deposited by magnetron sputtering; and 6, preparing the AlN thin film by using a magnetron sputtering technology.
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