WO2021028827A1

WO2021028827A1 - Gas sensor with capacitive micromachined ultrasonic transducer structure and functional polymer layer

Info

Publication number: WO2021028827A1
Application number: PCT/IB2020/057535
Authority: WO
Inventors: Dovydas BARAUSKAS; Darius Virzonis; Jonas BALTRUSAITIS; Gailius Vanagas; Donatas PELENIS
Original assignee: Kaunas University Of Technology
Priority date: 2019-08-14
Filing date: 2020-08-11
Publication date: 2021-02-18
Also published as: LT6821B; LT2019061A

Abstract

The invention is a gravimetric gas sensor for the selective detection of inorganic "acidic" gases, such as sulfur dioxide and carbon dioxide, in a gas mixture, without limiting the possibilities of use with other gases. The sensor consists of the structure and functional material of a capacitive micromachined ultrasonic transducer (CMUT). Known patents are close to this solution in that they are based on the measurement of changes in the resonant frequency of the CMUT structure. However, in order to selectively detect different gases, it is necessary to use different functional materials. The present invention is based on the use of functional polymers with specific properties, such as mPEI (methylated polyethyleneimine), without limiting the use of other polymers with suitable properties. The issue being addressed is more accurate, faster and more efficient gas detection and measurement of concentrations. The invention uses the cross-selectivity of the sensor for different gases, achieved by eliminating the need to modify the sensor with different materials, instead using a more complex measurement of the dynamic parameters of the detector. The structure of the CMUT is modified by functional material. Due to the interaction with the gas, the physical properties of the functional material (mass, modulus of elasticity) change, resulting in a change in the load parameters of the CMUT membrane that are measured. The invention can be applied by mining companies, and by a wide range of household consumers.

Description

GAS SENSOR WITH CAPACITIVE MICROMACHINED ULTRASONIC

TRANSDUCER STRUCTURE AND FUNCTIONAL POLYMER LAYER

FIELD OF THE INVENTION

The invention relates to sensors for materials, and, more particularly, to gravimetric gas sensors and meters that have resonators with a functional polymer-modified surface reacting with target gas molecules and altering the dynamic properties of the resonator.

BACKGROUND ART

Microelectromechanical systems (MEMS) are currently used as resonant, acoustic, gravimetric chemical or biochemical sensors. Capacitive Micromachined Ultrasonic Transducer (CMUT) structure consists of cells being capacitors with a single moving plate (membrane) separated from the structural base by a vacuum gap and insulating holders. Disc-shaped, square-shaped or polygon-shaped, perimeter-sealed membranes are used. The main structure of the transducer is formed on a silicon wafer or other suitable material, which is used as the background for the bottom electrode; the top electrode is mechanically combined with the moving plate. Due to Coulomb interaction, the moving plate bends toward the base when a voltage is applied to the electrodes. The vibration of the moving plate can be excited by an alternating electric field. CMUT elements may have one or more cells; the transducer consists of a set of elements. When the CMUT element is loaded with additional mass, for example, when coated with a polymer, it changes transducer’s dynamic parameters, such as resonant frequency and electromechanical impedance.

To date, a number of attempts are known that use electrostatically activated micromembrane CMUT structure for gas sensors.

CMUT transducers can have a single element, as well as one-dimensional and two-dimensional element arrays with the ability to simultaneously detect different species in multiple parallel channels, while still maintaining small transducer dimensions. High potential for parallel measurements occurs with a large number of measuring probes in a small area. Multidimensional and multi channel CMUT sensors are mentioned in the U.S. patent US9366651 B2.

For selective gas detection, the surface of CMUT sensors is modified with materials that specifically interact with the gases on interest by selectively adsorbing them on the modified surface, thus enabling the detection of physical and / or chemical changes in the modified surface. The ability to detect changes in mass due to adsorption of gas molecules is created by analyzing changes in resonant frequency. Modification of CMUT sensor membranes with functional materials and polymers is described in prior art patent sources WO201 1026836A1 , DE200910040052, US9366651 B2, and US7871569B2.

Typically, various materials and polymers are deposited on the surface of the CMUT sensor by drip coating, layer-by-layer deposition, or spraying methods. Also, the surface of the sensor can be modified by a spin-coating method, which allows reproducable layers of thin material to be obtained on the surface of the sensor. The coating of CMUT by spinning (spin-coating) is described in patents US7074634B2 [0167], US20070180916A1 , CN107151864B.

Patent application WO2019032938A1 describes a technology for the production of an optically transparent capacitive micromachined ultrasonic transducer (CMUT), and states in paragraph [0092] that it can be configured as a gravimetric sensor with a selective functional layer on a vibrating membrane, where the vibration frequency of the membrane goes down when the target molecules bind (or adsorb) on the functional layer. Such a sensor can be designed to operate in a gaseous or liquid medium and function as a gas sensor or biosensor. But real experiments, such described in the source Microchimica Acta, October 2014, Volume 181, Issue 13-14, pp 1749-175, DOI:10.1109/ULTSYM.2014.0646, show that under certain conditions, due to the specific interaction between the molecules of interest and the functional coating, the modulus of elasticity of the functional coating changes or the damping coefficient decreases, and as a result the resonant frequency may not only decrease, but even increase. Also, this patent application WO2019032938A1 does not disclose the use of CMUT to accurately and selectively measure concentrations of certain gases. Commonly available gas sensors on the market, such as electrochemical sensors, have shortcomings and often produce response to different gas mixtures without the ability to separate different gases according to the received signal, which limits the advantage of the information provided. The specificity (or cross- selectivity of a sensor) allows the selective detection of different gases in their mixture.

U.S. patent US8689606B2 (WO2010109363A2 / CN102362178B) describes a gas sensor chip having cells for transmitting and receiving ultrasound, configured for a sufficiently wide frequency range, and measuring the concentration of at least one of the gas components according to at least two responses in the range. The frequency range can be achieved by resizing the cell membranes, changing the bias voltage, and / or changing the air pressure for the cMUT or MEMS microphone set. A sensor chip can be used to detect CO2 gas. The chip is implemented as a stand-alone device that does not require separate sensors. The present invention uses an ultrasonic propagation time measurement that has a lower accuracy potential than selective gravimetric sensor. Also, the present invention does not use functional materials with their properties, so cross-selective gas detection is not possible.

Patent application WQ2011026836A1 (DE102009040052A1) describes a carbon dioxide sensor having a gas-sensitive layer made of linear polymer chains having side chains that have a primary amino group. The amino group reacts with CO2 to form a carbamate, and the physical properties of the substance change during this reaction. This change can be measured with a field effect transistor (FET), a Kelvin probe, or by measuring changes in capacitance or mass. Examples of materials are siloxanes, such as polyaminopropylmethyldiethoxyl, carbon nitride and cysteamine. The sensitive material can be mixed with a hydrophobic material or polymerized with hydrophobic monomers.

U.S. patent US9366651 B2 describes surface modifications of a sensor array, provides sensors with different molecular adsorption or binding functions. The first sensor in the array includes a first resonant element having a first surface that has a receptor material coated over the first base material. The second sensor includes a second resonant element having a second surface comprising a receptor material coated with a second base material that is different from the first base material. The first base material, the second base material, and the receptor material are selected in such a way that the first resonant element having a combination of receptor and first base material has a different ability to adsorb or bind one or more masses. The second resonant element, which has a combination of receptor material with the second parent material, is analytical. Receptor materials include 3-aminopropyltrimethoxysiloxane (AMO) and propyltrimethoxysiloxane (PTMS).

U.S. patent US7871569B2 describes systems and methods for detecting biomolecules in a sample using resonators with biosensors whose surfaces have the functionality to react with target biomolecules. In the embodiment, the device comprises a piezoelectric resonator, the functionalized surface of which is configured to react with the target molecules, thereby varying the mass and charge of the resonator, which changes the resonator frequency response accordingly. The resonator frequency response affected by the sample is compared to reference parameters, such as frequency response before exposure to the sample, retained initial frequency response, or control resonator frequency response.

The closest to the present invention prior art sources are patents US9366651 B2 and US7871569B2. However, they do not describe how to selectively and accurately measure certain gases, for example, concentrations of carbon dioxide and sulfur dioxide in the gas mixture. These prototype patents also use other types of resonators (such as quartz crystal microbalance (QCM) or other resonators based on piezoelectric crystal properties) with a sensitivity potential lower than that of the CMUT structure. Therefore, it is an object of the present invention to find functional polymers and measurement methods that allow accurate measurement of certain gas concentrations by CMUT sensors with a single CMUT transducer (or transducers of the same type connected in parallel).

SUMMARY OF THE INVENTION

The invention solves the problem of selective gas detection in a gas mixture by one CMUT transducer. The developed system allows to gravimetrically measure and analyze the amounts of different gases in the gas mixture. In one embodiment of the invention, the gas mixture is carbon dioxide and sulfur dioxide (in nitrogen). Also, the invention increases the amount of information obtained about interactions between gas molecules and functional materials. This improves the reliability of the measurement channel and reduces uncertainties, enriches the sensor signal information, detects ongoing specific interactions with the sensor cells. The invention uses imine group polymers for selective gas detection.

The embodiment of the experiment of the invention comprises an optimized system for detecting and measuring the concentration of carbon dioxide and sulfur dioxide in a gas mixture that is comprised of: · The CMUT sensor array with the surface modified by a polymer methylated polyethyleneimine (mPEI), where the surface is coated with a polymer by spin-coating;

• The electronic part, comprising measuring electronics circuit consisting of power supply electronics and the oscillator whose frequency is determined by the CMUT structure;

• The measurement data digitization and digital processing software.

In the embodiment, the system consists of an array of CMUT sensors modified by a thin layer of polymer and connected to the bias-voltage-generating electrical circuit for operating point optimization. The frequency and electromechanical impedance amplitude characteristics of the CMUT are monitored using the oscillator circuit that is powered by the separate power supply connected to the auxiliary voltage source. The amplified signal and its parameters, including the resonant frequency and the amplitude of the oscillator output signal, are digitized by the oscilloscope and transmitted to the computer via serial-format digital interface for digital processing, storage, and display.

The embodiment is used for the selective measurement of a mixture of carbon dioxide and sulfur dioxide (in nitrogen) with CMUT sensors, for the use of these sensors in the analysis of gas mixtures, and for the accurate determination of the concentrations of the mixture of carbon dioxide and sulfur dioxide. BRIEF DESCRIPTION OF DRAWINGS The accompanying diagrams and drawings form an integral part of the description of the invention and are provided as a reference to a possible embodiment of the invention, but are not intended to limit the scope of the invention. The drawings and diagrams do not necessarily correspond to the scale of the components of the invention. If some components are not necessary for explaining the operation of the invention and are not relevant, they are not provided.

Fig. 1 shows the schematic diagram of the single cellof the CMUT sensor. The following items are numbered in the figure: 101 - CMUT cell structure; 102 - the insulation layer; 103 - the top electrode contact pad; 104 - the top electrode; 105 - the surface functional polymer layer; 106 - the vacuum gap; 107 - the membrane; 108 - the bottom electrode contact pad; 109 - the bottom electrode.

Fig. 2 marked as 200, is a structural diagram of the measuring system. The following items are numbered in the figure: 201 - the bias voltage generator; 202 - the CMUT sensor; 203 - the surface modification; 204

- the oscillator circuit; 205 - the oscillator circuit power supply; 206 - the oscilloscope; 207 - the computer.

Fig. 3 shows the structural diagram of the fully assembled CMUT sensor. The following items are numbered in the figure: 301 - the CMUT sensor; 302

- the printed circuit board (PCB); 303 - the surface functional polymer; 304 - the top electrode (gold); 305 - the CMUT cell; 306 - the CMUT cell array; 307 - the epoxy resin layer; 308 - the contact pads; 309 - the bottom electrode contact pad; 310 - the gold connection wires; 311 - the PCB track for bottom electrode; 312 - the bottom electrode / ground contact; 313 - the PCB track for top electrode; 314 - the top electrode contact pad.

Fig. 4 marked as 400, is the resonant frequency shift and the amplitude of the oscillator output signal as a function of time: before the interaction, during CO2 and SO2 interaction, and after the interaction. The following items are numbered in the figure: 401 - the initial frequency; 402 - the frequency change during the interaction with CO2; 403 - the frequency change during the interaction with CO2 and SO2; 404 - the frequency change after interaction with CO2 and SO2; 405 - the initial amplitude; 406 - the starting time of CO2 interaction ; 407 - the amplitude during the interaction with CO2 and SO2; 408 - the starting time of CO2 and SO2 reaction; 409 - the end time of CO2 and SO2 interaction . Fig. 5 depicts the change of characteristics of the resonant frequency Frez of the CMUT membrane (107) coated with the layer of mPEI polymer, depending on the target gas in the test gas mixture: 501 - the concentrations of carbon dioxide CO2, 502 - the concentrations of sulfur dioxide SO2 at ambient temperature 23°C. DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses the CMUT sensor with modified surface for the selective detection of gas concentrations of two gases in a mixture, such as carbon dioxide and sulfur dioxide, using additional electronic components and measurement channels. The membrane surface of the CMUT sensor is modified with imine group polymers, which can basically be suitable for the selective detection of gases other than carbon dioxide and sulfur dioxide in the gas mixture.

Design of the sensor. The CMUT sensor (301) is comprised of at least one element (306) comprising at least one cell (101 , 305) having diameter of 42 pm, the number of cells per element (306) being 1600. Each cell includes the membrane (106) which is separated from the cell structural base (109) by insulating holders (102) to create a vacuum gap (106). The membrane top layer (104) is made of a material that has high electrical conductivity and good adhesion to the functional material, such as gold, on which the active functional material layer (105, 305) can be technologically formed to increase the selectivity of the transducer in the gas mixture. Other layers of materials can also be used that combine the electrical, mechanical, morphological, and other properties of the membrane and the functional layer as needed. The membrane can be disc shaped, square-shaped, polygon-shaped, or other. At least one membrane layer is electrically conductive and is used as the top electrode (104, 105, 304). The insulating layer (102), the electrode separation (108) and the vacuum gap (106) are formed between the membrane (107) and the substrate and is necessary for the membrane vibration. The sensor cells (101, 305) are formed on highly dopedsilicon wafer (109), which is used as the bottom electrode (308). A voltage is applied to the top electrode (104, 304) and the bottom electrode (109, 309). The membrane (107) bends towards the base due to the Coulomb interaction, and the vibration of the membrane is excited by changing the bending of the membrane by an alternating electric field. The voltage to the top and the bottom electrodes is supplied via special connectors (312, 314) constructed on the printed circuit board (302) whose tracks (311 , 313) are connected to the electrodes with gold wires (310) to the CMUT sensor contact pads (309, 304) coated with epoxy resin (307).

The CMUT sensor cells (101 , 305) are connected in parallel to form an array (306). The membrane array (306) is coated with gold (304) connecting all sensor cells (305) in parallel.

When the CMUT element is loaded with the additional mass, it changes transducer’s dynamic parameters: the resonant frequency and electromechanical impedance. In order for the CMUT sensor to act as the gas sensor, its cell surface is modified by an imine group polymer of the active substance (105, 303), such as methylated polyethyleneimine (mPEI) (303), which interacts with the target gas molecules that are explored. The surface modification (203) is performed by coating a thin layer of imine group polymer (e.g., mPEI) (303) on the sensor membranes (104, 106, 305) by spin-coating. As the gas molecules interact with the polymer layer, the dynamic characteristic of the sensor (301), the resonant frequency and electromechanical impedance, changes, , which determines the amplitude of the oscillator output signal. These variable parameters are recorded by the computer (207) to which the oscilloscope (206) is connected, receiving the real-time CMUT membrane resonant frequency and amplitude signal from the oscillator circuit (204) powered by a separate power supply (205). In this circuit, the sensor (301) acts as an electromechanical resonator. The operating point optimization is performed by bias-voltage-generating circuit (201). The computer stores real-time data about changes in the resonant frequency of the CMUT sensor (301) and the amplitude of the oscillator signal output as the sensor surface modification layer (303, 105) reacts with carbon dioxide and sulfur dioxide as the concentrations of this gas mixture change. Experimental system. The configured system (200) comprises the array of CMUT transducers (202) with a surface modified polymer (in this experiment: methylated polyethyleneimine or mPEI) coated on a CMUT sensor by spin-coating and the measuring electronics circuit consisting of power supply electronics, signal amplification electronics, use of the oscilloscope for data storage in conjunction with the user interface capable of detecting the concentration of carbon dioxide and sulfur dioxide in a mixture of these gases. This system consists of:

• The CMUT sensor array (202) modified with a thin polymer layer (203) connected to the bias-voltage-generating electrical circuit (201) for obtaining an optimal operating point;

• The CMUT frequency and signal amplitude characteristics are monitored using a Colpitts or other suitable type of oscillator circuit (204) that is powered by a separate power supply (205) connected to the auxiliary voltage source. · The amplified signal and signal parameters, including resonant frequency and amplitude, are transmitted by the oscilloscope (206) to the computer (207) via the serial connection and stored with the user interface program using a matched code that communicates with the oscilloscope (206) and stores the received data on the computer (207). · The computer (207) in the user interface software, performing the processing of the collected signal data, analyzing and determining the concentrations of the target gases (in the case of this experiment - carbon dioxide CO2 and sulfur dioxide SO2) in the test mixture according to the signal dynamics. Processing of the measurement data and the results. The results of the experiment are related to the cross-selective gas detection and measurement in real time, according to the Fig. 4, (400). Experiments with dry mixtures of nitrogen N2, carbon dioxide CO2, and sulfur dioxide SO2 gas were performed to determine changes in the membrane resonant frequency of the polymer mPEI-functionalized CMUT sensor and changes in the amplitude of the oscillator output signal. Firstly, the test chamber was vented by the flow of dry nitrogen N2 at a steady flow rate to bring the CMUT sensors to equilibrium. The initial resonant frequency (401) and signal amplitude (405) were measured in real time and recorded continuously throughout the experiment. When CO2 gas was introduced into the chamber due to the weak interaction of mPEI and CO2 molecules, a resonant frequency shift Ah - Ah was observed (402), with no obvious detectable changes in the electromechanical impedance (resistance) spectrum (406). When SO2 gas was introduced into the mixture, due to the strong chemical interaction of mPEI and SO2 molecules, changes in resonant frequency and signal amplitude were observed at the time (408) when SO2 gas was introduced and reacted with the mPEI layer, as shown in Figure 4. This phenomenon is related to a weak interaction of CO2 and mPEI, which does not change the physical structure of the polymer, however the strong chemical interaction of the functional polymer (mPEI) with SO2 molecules changes the polymer film properties, and this is seen in the diagram as the persistent change in resonant frequency (404), when SO2 gas does not remain in the test chamber (409). These changes increase the vibration damping coefficient and the membrane mass, which in turn results in a shift in the resonant frequency and a decrease in the amplitude of the oscillator output signal (404), which can be seen in Figure 4.

In the present invention it is important that the results seen in Figure 4 show that by evaluating the changes in both resonant frequency and electromechanical impedance that directly determine the amplitude of the oscillator output signal, a mixture of two "acidic" gases, SO2 and CO2, can be detected and the effect of each can be separated and rejected.

Also, the experiments performed in the present invention with the functional coatings of the gravimetric sensor and different gases show that the resonant frequency of the membrane (107) can not only decrease but also increase. Due to the specific interaction between the gas molecules and the functional coating, not only does the membrane mass rn_m increase with additional mass rm_a, but the modulus km of elasticity of the membrane with functional coating may change by additional value k_e or the damping coefficient may decrease, so the membrane resonant frequency Frez may increase according to equation: This is observed by measuring not only the resonant frequency of the membrane (Frez), but also the resonance quality (Q) or amplitude of the oscillator output signal due to the membrane mass rn_m and the damping coefficient b_api, the value of witch depends on the factors of the measurement environment such as temperature, pressure and humidity.

Combinations of the dynamic changes of Frez and Q of these two parameters are possible when using CMUT functional coatings and testing different gases. Differentiation of dynamic changes is possible according to the positive change (+AF_rez, + DO), negative change (-AFrez, -AQ) or no change (AFrez = 0, AQ = 0) of any parameter - thus 8 non-zero combinations of changes allowing to identify different gaseous reagents and / or their states are derived. In this way, changes in these parameters due to different gas interactions can be cross- selectively separated and rejected.

By recording the differences in the CMUT sensor signal at different gas concentrations and ambient conditions, it is also possible to establish reference characteristics of the interaction between target gas and functional coating, which can then be used to detect and measure the target gas concentration in a gas mixture of unknown composition. Example of such characteristic is shown in Fig. 5, and it shows the resonant frequency Frez of the CMUT membrane (107) coated with the mPEI polymer layer as a function of the concentration of carbon dioxide CO2 (501 ) and the concentration of sulfur dioxide SO2 (502) in the test gas mixture at ambient temperature 23°C. These characteristics (501 ) and (502) show that the interactions of CO2 and SO2 with the functional coating differ from each other, and from these interactions, the concentration of the target gas in the test mixture can be estimated by analyzing the dynamic changes in the CMUT signal and comparing it with the reference characteristics. The described embodiment and experiment with SO2 and CO2 gases is an integral part of the present invention, but is not intended to limit the scope of the invention. Aspects of the invention, such as the use of imine group polymers in the membrane functional layer, the cross-selectivity of the CMUT sensor for different target gases, and their detection and determination of concentrations by analyzing the dynamic parameters of the CMUT sensor signal, can be used for other target gas detection and concentration measurements. Also, the described methods of digital processing of a CMUT sensor signal are an integral part of the present invention, but should not limit the scope of the invention: various other digital signal processing methods and algorithms can be used in the invention to increase the cross-selectivity of a single CMUT sensor with functional coating for different target gases and their concentrations in the mixture.

Claims

1. Gravimetric gas sensor comprising

- at least one capacitive micromachined ultrasonic transducer (CMUT) (101) with the membrane (107), the bottom base electrode (109) and the top membrane electrode (104),

- the functional polymer layer (105) covering the CMUT transducer membrane (107),

- the electrical control circuit of CMUT transducer, the oscilloscope and the computer for storage and processing of measurement data (200), ch aracte ri zed in that the functional polymer layer (105) covering the CMUT transducer membrane (107) is an imine group polymer for accurate measurement of gas concentrations with one CMUT transducer (101) or several CMUT transducers (101) of the same type connected in parallel.

2. The gas sensor according to claim ^characteri zed in that the CMUT transducer (101 ) has an array (301 ) consisting of a topology of more than one

CMUT element, in which the electrodes of the CMUT elements (101) are connected in parallel.

3. The gas sensor according to claim 1, characteri zed in that the membranes (107) of the CMUT sensor elements are coated with the functional polymer (105) by spin-coating.

4. The gas sensor according to claim ^characteri zed in that it uses the cross-selectivity of the functional polymer (105) for at least two different target gases in the test gas mixture.

5. The gas sensor according to claim 4, ch aracte ri zed in that the target gas is detected in the test gas mixture and its concentration is determined by analyzing (400) the resonant frequency Frez and the resonant quality Q of the dynamic parameters of the CMUT signal membrane (107).

6. The gas sensor according to claim 1, characteri zed in that the functional polymer (105) is an imine group methylated polyethyleneimine (mPEI).

7. The gas sensor according to claim 6, characteri zed in that it is used to measure concentration of carbon dioxide in a gas mixture (402).

8. The gas sensor according to claim 6, characteri zed in that it is used to measure concentration of sulfur dioxide in a gas mixture (404).

9. The gas sensor according to claim 6, characteri zed in that it is used to detect and to measure concentration of sulfur dioxide and carbon dioxide in a gas mixture (403).