On-chip microfluidic trace H2 sensor and preparation method thereof

Abstract

The invention discloses an on-chip microfluidic trace H2 sensor and a preparation method thereof, and belongs to the technical field of gas sensing. According to the method, a Pd film is grown between two layers of MOF films, and the three layers of composite films are self-curled into a tubular structure, so that pipeline type rapid detection of on-chip microfluidic trace H2 is realized. By means of the micropore structure of the MOF thin film, H2 target molecules with the small molecular size selectively penetrate through the MOF thin film, other gas molecules larger than micropores are isolated outside, and sensitive detection and quick response to low-concentration H2 are achieved.

Description

Technical Field

[0001] The present invention relates to a microfluidic trace H 2 sensor on a chip and a preparation method thereof, belonging to the technical field of gas sensing. Background Art

[0002] Hydrogen (H 2 ) is regarded as the next-generation clean energy due to its high energy combustion efficiency and the combustion product being only water (H 2 O). However, when H 2 is used as a green fuel, the following problems are faced: First, H 2 itself is colorless and odorless, making it difficult to detect; Second, the minimum ignition energy of hydrogen is extremely low, only 0.017 mJ, which means that even very small energies, such as electrostatic discharge, electric sparks of electrical equipment, etc., are sufficient to ignite hydrogen; Third, the lower explosive limit concentration of H 2 in air is only 4%, and major accidents are extremely likely to occur due to concentration accumulation. Therefore, highly sensitive sensors for detecting leaked H 2 are extremely important.

[0003] Among various H 2 sensors, Pd-based resistive H 2 sensors have received great attention due to their simple principle, low cost, and sufficient sensitivity. Currently, Pd-based H 2 sensors mainly have two sensing detection mechanisms: One is based on the reaction of Pd with H 2 to form PdH x and detect by increasing the resistance; The other uses the phase change of α-Pd reacting with H 2 to form β-PdH x to cause volume expansion to detect H 2 in the range of 1-2%. It should be noted that the sensing characteristics of current these H 2 sensors are evaluated in a nitrogen atmosphere, rather than in normal air. This is because there are various gases in the air, and the sensing performance of Pd will be significantly reduced due to the presence of interfering gases such as O 2 , SO 2 and H 2 S. For example, in air, chemically adsorbed O (Pd-O) reacts with H 2 adsorbed on the Pd surface to generate water.

[0004]

[0005] H 2 O(ads)→H 2 O(gas)

[0006] These reactions reduce the steady-state coverage of chemisorbed H on the Pd surface, resulting in PdH x with a reduced H absorption rate and a lower steady-state value of x. In addition, oxygen adsorbed on the Pd surface blocks the H adsorption sites. For these reasons, Pd-based H 2 sensors often exhibit poor H sensing sensitivity and slow reaction kinetics in ambient air.

[0007] To minimize the shielding effect of O 2 on Pd-based sensors, Li from the University of California, USA proposed functionalizing the Pd surface with a Pt catalyst. The Pt catalyst accelerates the removal of O 2 from the Pd surface in the form of water, achieving enhanced H 2 detection performance. However, due to the activation of the Pt catalyst, the sensor needs to operate at a high temperature (100 °C), limiting the detection of H 2 at room temperature. On the other hand, current commercial H 2 sensors are relatively large in size and require a large amount of target gas to cover the sensing material surface for concentration monitoring. Due to the sensing mechanism, the miniaturization and integration of sensors are far from sufficient, and there is an urgent need for on-chip microfluidic H 2 sensitive sensing. Summary of the Invention

[0008] To solve the existing problems, the present invention considers that metal-organic frameworks (MOFs) are coordination polymers with a periodic network structure formed by metal ions and organic ligands, and their surfaces have abundant micropores below 2 nanometers. Using the MOF thin film as a nanofilter membrane, gas molecules larger than the micropore size of the thin film in air molecules can be filtered, while small-sized H 2 molecules can pass through quickly without obstruction. Combining with the Pd sensing material can achieve efficient sensing detection of low-concentration H 2 and avoid interference from other gas molecules. Therefore, an on-chip microfluidic trace H 2 sensor and its preparation method are provided. During preparation, the on-chip Pd thin film and the MOF thin film are curled into a three-dimensional microtube structure, realizing rapid and sensitive detection of microscale H 2 in the microtube channel.

[0009] The technical solution of the present invention is to grow the Pd thin film between two layers of MOF thin films, and self-curl the three-layer composite film into a tubular structure to achieve pipeline-type rapid detection of on-chip microfluidic trace H 2 . Utilizing the microporous structure of the MOF thin film, selectively permeate the target H 2 target molecules with small molecular sizes, while isolating other gas molecules larger than the micropores, achieving sensitive detection and rapid response to low-concentration H 2 . The specific steps are as follows:

[0010] Step (1), deposit a metal layer as an electrode on the cleaned substrate through photolithography and coating processes;

[0011] Step (2), deposit a layer of nanometer film on the metal layer by coating method as a sacrificial layer;

[0012] Step (3), deposit the first layer of ZnO nanometer film by atomic layer deposition at high temperature on the sacrificial layer;

[0013] Step (4), deposit a layer of Pd nanometer film by coating method on the high-temperature ZnO nanometer film;

[0014] Step (5), deposit the second layer of ZnO nanometer film by atomic layer deposition at low temperature on the Pd nanometer film again;

[0015] Step (6), divide the ZnO / Pd / ZnO composite film into independent patterns by photolithography;

[0016] Step (7), remove the sacrificial layer by etching method, release the upper composite film, and realize the self-curling of the film; the composite film is a film formed by growing a Pd film between two layers of ZnO films;

[0017] Step (8), perform critical point drying on the self-curled microtube to avoid the collapse of the tubular structure;

[0018] Step (9), in the heating system, place the microtube directly above the MOF precursor, set the appropriate temperature for the system, and convert the ZnO nanometer film in the microtube into a MOF film.

[0019] In step (1):

[0020] The coating process includes electron beam evaporation, magnetron sputtering, atomic layer deposition, sol-gel, physical vapor deposition, and chemical vapor deposition.

[0021] The metal layer includes Cu, Al, Ag, Au, Ni, Ti, and preferably Au is used as the electrode layer.

[0022] The substrate includes a single crystal silicon substrate, a silicon dioxide (glass) substrate, a metal substrate, and a ceramic substrate.

[0023] In step (2):

[0024] The coating method includes electron beam evaporation, magnetron sputtering, atomic layer deposition, sol-gel, physical vapor deposition, and chemical vapor deposition.

[0025] The nanometer film used as the sacrificial layer includes a Ge nanometer film, a photoresist layer, SiO 2Types of sacrificial layers commonly used in nanometer thin films and other semiconductor processes.

[0026] The thickness range of the sacrificial layer is 5 - 500 nm, and the preferred thickness is 20 - 100 nm.

[0027] In step (3):

[0028] For the high-temperature deposition, the deposition temperature is 150 °C - 500 °C, and the preferred temperature is 200 °C - 300 °C.

[0029] For the first layer of ZnO nanometer thin film, the number of atomic layer deposition cycles is 250 - 1000, and the preferred number of cycles is 300 - 600.

[0030] In step (4):

[0031] The coating methods include electron beam evaporation, magnetron sputtering, atomic layer deposition, sol-gel, physical vapor deposition, and chemical vapor deposition.

[0032] The thickness range of the layer of Pd nanometer thin film is 5 - 200 nm, and the preferred thickness is 10 - 30 nm.

[0033] In step (5):

[0034] For the low-temperature deposition, the deposition temperature is 30 °C - 120 °C, and the preferred temperature is 50 °C - 100 °C. Ensure the temperature difference between the two depositions of ZnO nanometer thin film, so that the formed composite thin film has a high stress difference, and the composite thin film can be driven to self-curl after the film is released.

[0035] For the second layer of ZnO nanometer thin film, the number of atomic layer deposition cycles is 50 - 800, and the preferred number of cycles is 100 - 400, and the number of cycles is at least 200 less than that of the ZnO nanometer thin film deposited at high temperature.

[0036] In step (6):

[0037] The lithography process includes spin-coating photoresist, exposure, development, etching the composite thin film, removing the photoresist with acetone, and cleaning.

[0038] The independent patterns include cross-shaped, rectangular, circular, triangular, star-shaped, and other regular and irregular patterns.

[0039] In step (7):

[0040] The etching methods include dry etching and wet etching.

[0041] The dry etching includes XeF 2 gas etching of Ge sacrificial layer, CF 4 gas etching of SiO 2Sacrificial layer and the method of directional gas corrosion of other types of sacrificial layers.

[0042] The wet etching described above includes H 2 O 2 solution for etching Ge sacrificial layer, HF solution for etching SiO 2 sacrificial layer and the method of directional solution corrosion of other types of sacrificial layers.

[0043] For the release of the upper composite thin film, the release degree of the pattern is adjusted to be 50% - 90% by adjusting the etching time, and preferably the release degree is 70% - 90%.

[0044] In step (9):

[0045] The heating system described above includes a CVD system, a tube furnace, a constant temperature oven, and other temperature control devices.

[0046] The MOF precursors described above include ligands for synthesizing ZIF-8: 2-methylimidazole; ligands for synthesizing ZIF-4: imidazole; ligands for synthesizing ZIF-90: imidazole-2-carboxaldehyde; and ligands for synthesizing other Zn-based MOFs.

[0047] The suitable temperatures described above are 160°C - 200°C for the conversion to ZIF-8, 100°C - 140°C for the conversion to ZIF-4, 200°C - 230°C for the conversion to ZIF-90, and the suitable temperatures for synthesizing other Zn-based MOFs.

[0048] The beneficial effects of the present invention are:

[0049] (1) The method for Pd to detect H 2 proposed by the present invention is different from the existing method of directly detecting H by Pd. 2 Specifically, a Pd thin film is coated with two layers of MOF thin films, and other gas molecules except H 2 are blocked by the inherent micropores of the MOF thin films, realizing sensitive and rapid sensing of H 2 ;

[0050] (2) The method for Pd to detect H 2 proposed by the present invention is different from the existing Pd structures such as planar thin films, particles, and nanowire structures. The Pd thin film is curled into a three-dimensional tubular structure, which can realize on-chip multi-channel microfluidic detection and is convenient for compatibility with modern semiconductor integrated devices;

[0051] (3) The method proposed in the present invention, which first forms a microtube structure from a self-curling ZnO / Pd / ZnO composite thin film and then converts it into a MOF / Pd / MOF microtube by gas-phase transformation, realizes the regulation of the three-dimensional tubular structure of the composite thin film. This is essentially different from the existing planar MOF thin films and devices, breaking through the application field of MOF thin films from two-dimensional morphology to three-dimensional morphology.

[0052] (4) The method proposed in the present invention for using the micropores of porous materials to block interfering gases has great reference value for the detection of other types of gases. The proposed method for preparing MOF microtubes has great reference value for the three-dimensional application of MOF thin films.

[0053] (5) In the present invention, two ZnO nanometer thin films with a stress difference are formed on both sides of the Pd nanometer thin film by high-temperature atomic layer deposition and low-temperature atomic layer deposition respectively, so as to realize a self-curling microtube structure by using the stress difference between the two ZnO nanometer thin films, without the need to additionally introduce a strain driving layer with a prestress gradient. As a result, the finally transformed three-dimensional tubular structure only contains the MOF thin film for blocking other gas molecules except the H 2 target molecule and the Pd thin film for detecting the H 2 target molecule, realizing sensitive detection and rapid response to low-concentration H 2 . Brief Description of the Drawings

[0054] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0055] Figure 1 It is a schematic structural diagram of a ZIF-8 / Pd / ZIF-8 curled tubular H 2 sensor.

[0056] Figure 2 It is a schematic diagram of the layered structure of the film on the pre-release curled sheet.

[0057] Figure 3 It is a detection curve graph of the ZIF-8 / Pd / ZIF-8 curled tubular on-chip microfluidic sensor for different concentrations of H 2 .

[0058] Figure 4 It is a corresponding curve of the change in the resistance value of the ZIF-8 / Pd / ZIF-8 curled tubular on-chip microfluidic sensor and the H 2 concentration.

[0059] Reference numerals in the figure: 1 is a Si wafer, 2 is an electrode, 3 is a sacrificial layer, 4 is a ZnO nanometer thin film deposited at high temperature, 5 is a Pd nanometer thin film, 6 is a ZnO nanometer thin film deposited at low temperature, and 7 is a coiled microtube. Specific implementation mode

[0060] To make the objectives, technical solutions and advantages of the present invention clearer, the following will further describe the implementation modes of the present invention in detail with reference to the accompanying drawings.

[0061] Example 1

[0062] This example provides a microfluidic trace H 2 sensor, as Figure 1 shown, the overall shape is coiled tubular; Figure 2 It is a schematic diagram of the layered structure of the thin film on the chip before releasing the coil. From bottom to top, they are the substrate layer 1, the electrode layer 2, the sacrificial layer 3, the high-temperature ZnO nanometer thin film layer 4, the Pd nanometer thin film layer 5, and the low-temperature ZnO nanometer thin film layer 6.

[0063] The preparation method of this microfluidic trace H 2 sensor is as follows:

[0064] Step (1) Prepare the substrate layer 1: Thoroughly clean the cut silicon substrate by means of acetone, ethanol, and deionized water;

[0065] Step (2) Prepare the electrode layer 2: Prepare a 50-nm-thick patterned Au layer as the electrode on the silicon substrate by magnetron sputtering;

[0066] Step (3) Deposit the sacrificial layer 3: Deposit a 50-nm-thick Ge nanometer thin film as the sacrificial layer on the electrode layer by electron beam evaporation, and the deposition rate

[0067] Step (4) Deposit the high-temperature ZnO nanometer thin film layer 4: Deposit 400 cycles of ZnO nanometer thin film on the Ge nanometer film layer by atomic layer deposition. The deposition temperature is 250 °C. The precursors for atomic layer deposition of ZnO nanometer thin film are diethyl zinc and deionized water respectively. The cycle process is deionized water pulse (35 ms), waiting (5 s), N 2 purge (30 s), diethyl zinc (30 ms), waiting (5 s), N 2 purge (30 s);

[0068] Step (5) Deposit the Pd nanometer thin film layer 5: Deposit a 20-nm-thick Pd nanometer thin film on the ZnO nanometer thin film by electron beam evaporation at a deposition rate of ;

[0069] Step (6) Depositing the low-temperature ZnO nanothin film layer 6: Deposit the second layer of ZnO nanothin film with 200 cycles again on the upper layer of the Pd nanothin film by atomic layer deposition, and the deposition temperature is 60 °C;

[0070] The high-temperature ZnO nanothin film, Pd nanothin film, and low-temperature ZnO nanothin film deposited in the above steps (4) to (6) form a composite film;

[0071] Step (7) Patterning the composite film layer: Spin-coat a photoresist layer with a thickness of about 2 μm by photolithography process, and then pattern the photoresist layer into independent rectangles with a length of 5 μm and a width of 1 μm by laser direct writing lithography. After that, etch away the exposed ZnO nanomembrane with diluted hydrochloric acid (5 mM), and then clean the photoresist with acetone, and clean with ethanol and deionized water.

[0072] Step (8) Releasing the sacrificial layer 3: Immerse the sample in a solution containing 30% H 2 O 2 at 60 °C for 15 min, corrode and remove the Ge sacrificial layer to realize the self-curling of the ZnO / Pd / ZnO composite film.

[0073] Step (9) Drying: Use a critical point dryer to dry the self-curled ZnO / Pd / ZnO microtubes to avoid the collapse of the tubular structure.

[0074] Step (10) Gas-phase conversion: Place the ground 2-methylimidazole in the thermal center of the tube furnace cavity, and place the ZnO / Pd / ZnO microtube sample 5 cm directly above the 2-methylimidazole powder. Use high-purity N 2 as the carrier gas, maintain a gas flow rate of 80 sccm, and grow at a heat source temperature of 180 °C for 6 h, then the ZnO / Pd / ZnO microtubes can be converted into ZIF-8 / Pd / ZIF-8 microtubes, and a microfluidic micro H 2 sensor on the chip can be obtained.

[0075] Using the ZIF-8 / Pd / ZIF-8 curled tubular microfluidic micro H 2 sensor prepared by the above steps for detecting different concentrations of H 2 , the detection results are as Figure 3 shown. It can be seen that when the H 2 concentrations are 0.2%, 0.5%, and 1% respectively, the resistance values all change greatly, indicating that the sensor can be used to detect the H 2 concentration.

[0076] Figure 4 As shown for H 2Calibration curve of the change in concentration and resistance value. Therefore, the device has a high sensitivity of 1.25 kΩ per one percent concentration. The high sensitivity benefits from the blocking of interfering gas molecules by the MOF thin film (here, the ZIF-8 thin film), and the low concentration actual detection limit (<0.2%) benefits from the coiled microtube structure. Compared with the planar structure, the coiled microtube structure has a larger contact area with H 2 and thus has a lower actual detection limit.

[0077] Example Two

[0078] In this example, the on-chip microfluidic trace H 2 sensor was prepared according to the method of Example One, except that the sacrificial layer was a SiO 2 nano-thin film.

[0079] A 500-nm-thick SiO 2 nano-thin film was grown as the sacrificial layer on the electrode layer by thermal oxidation. Then, the high-temperature ZnO nano-thin film, Pd nano-thin film, and low-temperature ZnO nano-thin film were deposited in sequence according to the method of Example One, and the composite thin film was patterned by photolithography. In the process of step (8) release, the SiO 2 sacrificial layer was etched by soaking in a 10% hydrofluoric acid solution for 3 min to obtain a self-coiled microtube structure. The subsequent drying and gas-phase conversion of MOF were the same as those in Example One.

[0080] Example Three

[0081] In this example, the on-chip microfluidic trace H 2 sensor was prepared according to the method of Example One, except that the deposition methods of the Ge nano-thin film in step (3) and the Pd nano-thin film in step (5) were magnetron sputtering.

[0082] The Ge nano-thin film and Pd nano-thin film were deposited by magnetron sputtering with a sputtering power of 200 W. The film thicknesses were the same as those in Example One, which were 50 nm and 20 nm respectively.

[0083] Example Four

[0084] In this example, the on-chip microfluidic trace H 2 sensor was prepared according to the method of Example One, except that the temperatures of the two atomic layer depositions of ZnO thin film in steps (4) and (6) were 300 °C and 80 °C respectively.

[0085] Step (4) was to deposit 400 cycles of ZnO nano-thin film on the Ge nano-thin film by atomic layer deposition at a deposition temperature of 300 °C. Step (6) was to deposit 200 cycles of the second ZnO nano-thin film on the Pd nano-thin film again by atomic layer deposition at a deposition temperature of 80 °C.

[0086] Example 5

[0087] This example prepares the on-chip microfluidic trace H 2 sensor according to the method of Example 1, except that the number of cycles of atomic layer deposition of high-temperature and low-temperature ZnO nanofilms in steps (4) and (6) are 300 and 100 respectively.

[0088] Step (4) is to deposit ZnO nanofilm for 300 cycles at 250 °C on the Ge nanofilm by atomic layer deposition. Step (6) is to deposit the second layer of ZnO nanofilm for 100 cycles at 60 °C on the Pd nanofilm by atomic layer deposition. The thickness of the second layer of atomic layer deposited ZnO nanofilm should not be too large, that is, it cannot exceed the first layer of ZnO nanofilm, otherwise the self-curling of the multi-layer composite film cannot be achieved.

[0089] Example 6

[0090] This example prepares the on-chip microfluidic trace H 2 sensor according to the method of Example 1, except that the lithography pattern in step (7) is designed as a cross shape.

[0091] The middle block of the cross pattern can provide stress for the edge patterns, and the rectangles at each edge curl towards the middle of the cross. The finally formed on-chip microfluidic trace H 2 sensor has fluid channels in all directions and is suitable for special cases where the H 2 source direction is uncertain.

[0092] Example 7

[0093] This example prepares the on-chip microfluidic trace H 2 sensor according to the method of Example 1, except that the etching method in step (8) uses XeF 2 gas dry etching.

[0094] H 2 O 2 The liquid-phase etching of the H 2 O solution is inexpensive and environmentally friendly, but the etching speed is slow and pinholes may appear in the film. While the dry etching using XeF

[0095] gas reacting with Ge has a faster etching speed and a higher yield, and is suitable for industrial production.

[0096] This example prepares the on-chip microfluidic trace H 2 sensor according to the method of Example 1, except that the MOF precursor in step (10) is selected as imidazole and the synthesis temperature is 120 °C.

[0097] The ground imidazole is placed at the thermal center of the tube furnace cavity, and the sample substrate with ZnO / Pd / ZnO microtubes is placed 5 cm directly above the imidazole powder. Growth is carried out at a heat source temperature of 120 °C for 6 h. ZnO and imidazole are synthesized into ZIF-4 at 120 °C. The inherent micropore sizes of ZIF-4 and ZIF-8 are slightly different and can be used for selective permeation of specific gases.

[0098] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing an on-chip microfluidic trace H2 sensor, characterized in that: The method comprises: Step 1, preparing a substrate layer; Step 2, preparing electrodes: depositing a metal layer as an electrode on the substrate layer through a coating process and a photolithography process; Step 3, depositing a sacrificial layer: depositing a nano-thin film as a sacrificial layer on the metal layer by using a coating process; Step 4, depositing a high-temperature ZnO nanofilm layer: depositing a first layer of ZnO nanofilm at high temperature on the sacrificial layer by atomic layer deposition; Step 5, depositing a Pd nanofilm layer: depositing a layer of Pd nanofilm on the high-temperature ZnO nanofilm by a coating method; Step 6, depositing a low-temperature ZnO nanofilm layer: depositing a second layer of ZnO nanofilm at low temperature on the Pd nanofilm again by atomic layer deposition, wherein the thickness of the second layer of ZnO nanofilm is less than that of the first layer of ZnO nanofilm; Step 7, patterning the composite thin film layer: using a photolithography process to divide the ZnO / Pd / ZnO composite thin film formed in steps 4 to 6 into independent patterns; Step 8, releasing the sacrificial layer: removing the sacrificial layer by an etching method, releasing the upper ZnO / Pd / ZnO composite film, achieving self-curling of the film, and forming ZnO / Pd / ZnO microtubes; Step 9, gas phase conversion: using a heating system to convert the ZnO nanofilm inside the ZnO / Pd / ZnO microtube into a MOF film, thereby converting the ZnO / Pd / ZnO microtube into a MOF / Pd / MOF microtube to obtain an on-chip microfluidic trace H2 sensor.

2. The method according to claim 1, characterized in that When depositing the high-temperature ZnO nano-film layer in step 4, the deposition temperature is 150°C to 500°C, preferably 200°C to 300°C.

3. The method according to claim 2, characterized in that When depositing the low-temperature ZnO nano-film layer in step 6, the deposition temperature is 30°C to 120°C, preferably 50°C to 100°C.

4. The method according to claim 1, characterized in that: The thickness of the sacrificial layer in step 3 is in the range of 5 to 500 nm, preferably in the range of 20 to 100 nm.

5. The method according to claim 1, characterized in that In step 5, the thickness of the Pd nanofilm layer ranges from 5 to 200 nm, preferably from 10 to 30 nm.

6. The method according to claim 1, characterized in that The material of the metal layer includes Cu, Al, Ag, Au, Ni, Ti, preferably Au.

7. The method according to claim 1, characterized in that The step 9 comprises: placing the ZnO / Pd / ZnO microtubes directly above the MOF precursor in the heating system, setting a corresponding suitable temperature according to the MOF precursor, and converting the ZnO nanofilm in the microtubes into a MOF film; wherein the MOF precursor is a ligand for synthesizing Zn-based MOF.

8. The method according to claim 1, characterized in that The nanofilm used as the sacrificial layer in step 3 includes a Ge nanofilm, a photoresist layer and a SiO2 nanofilm; The coating process includes electron beam evaporation, magnetron sputtering, atomic layer deposition, sol-gel, physical vapor deposition, and chemical vapor deposition.

9. The method according to claim 1, characterized in that: After step 8 and before step 9, a drying step is further included to perform critical point drying on the ZnO / Pd / ZnO microtubes formed in step 8 to avoid collapse of the tubular structure.

10. An on-chip microfluidic trace H2 sensor, characterized in that: The sensor comprises a three-dimensional tubular structure formed by a MOF / Pd / MOF composite thin film layer, and is prepared by the method according to any one of claims 1 to 9.

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