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Graphene is a new type of nanomaterial that has emerged in recent years and can be applied to the field of pressure detection. At present, a variety of detection methods based on capacitive, resistive and tunnel current are used in graphene pressure sensors.
As a sensitive unit of pressure sensors, graphene nanofilms are the core factors affecting the performance of the device. Y.X. Yang et al. confirmed that graphene is susceptible to pollution and adulteration by the external environment during the transfer process, especially water molecules and dust in the air, resulting in the electrical performance of graphene devices not meeting the expected design indicators, and unable to give full play to the excellent mechanical and electrical properties of graphene.
At present, high-density organic matter is mostly used as a protective layer to isolate the influence of the external environment on graphene. H.K. Seo et al. use PDMS and PET as protective layers of flexible graphene composite film layers, extending the life of the device from 3h to 70h.
There is still a gap with the practical requirements. C. Yan et al. made a graphene coating layer based on P4VP and PET, and the results showed that the coating layer had a certain protective effect on graphene.
However, the change of graphene resistance reaches 200%, which is unacceptable in the calibration and commercialization of actual sensors. In general, the current graphene devices cannot meet the needs of practical applications.
Boron nitride is a kind of nano-ceramic thin film material, which is the same hexagonal lattice structure as graphene, and the lattice mismatch is only 1.8%, and the heterostructure formed by the two is stable and the bonding force of each film layer is strong. The surface flatness of boron nitride film is two orders of magnitude higher than that of silica, so it is very suitable as a substrate for graphene, and a flatter substrate surface will effectively reduce the surface phonon scattering of graphene, increase the mean free path of carriers, and improve the electrical properties of graphene.
At the same time, boron nitride can effectively block the pollution and doping of graphene by oxygen molecules, dust, ions, etc. in the surrounding environment. The high-quality preparation process of boron nitride/graphene/boron nitride heterojunction using boron nitride as a protective layer of graphene and studying boron nitride/boron nitride heterojunction can provide new ideas for improving the performance of graphene devices.
Therefore, this paper focuses on the preparation process of boron nitride/graphene/boron nitride heterojunction, synthesizes the heterojunction by high-quality graphene and boron nitride transfer process, and conducts necessary tests on the manufactured samples, and the test results verify the feasibility and effectiveness of the process scheme.
1 The working principle of boron nitride/graphene/boron nitride heterojunction
1.1 Structure of boron nitride/graphene/boron nitride heterojunction
Boron nitride/graphene/boron nitride heterojunctions are composed of substrates, electrodes and nanofilm layers. The substrate is composed of a silicon substrate and a silica dielectric layer, etched in the silica dielectric layer to form a pressure chamber, the electrodes are located on both sides of the pressure chamber, and the pressure chamber is covered with boron nitride/graphene/boron nitride nanofilm, and the film is interconnected with the electrodes on both sides of the pressure chamber. The structure of boron nitride/graphene/boron nitride heterojunction is shown in Figure 1.
Figure 1 Schematic diagram of boron nitride/graphene/boron nitride heterojunction
1.2 Working principle of boron nitride/graphene/boron nitride heterojunction
Boron nitride/graphene/boron nitride heterojunctions convert the graphene strain caused by external pressure into resistance changes of the heterostructure through the piezoresistive effect of graphene, so as to realize the mechanical sensing of external pressure signals.
When the boron nitride/graphene/boron nitride heterojunction work, the nanofilm and the external power supply and signal acquisition system constitute a loop, the heterojunction is placed in the pressure environment to be measured, when the pressure value in the pressure environment to be measured and the standard atmospheric pressure value sealed in the pressure chamber produce a pressure difference, the graphene film is strained, and its conduction band and valence band open the energy gap at the Dillac point, thereby affecting the conductivity of graphene.
Finally, the graphene resistance changes, and the voltage change at both ends of the film in the circuit can be detected by the signal acquisition system. The working principle of graphene mechanical sensitive elements is shown in Figure 2.
Figure 2 Schematic diagram of the working principle of boron nitride/graphene/boron nitride heterojunction
2 Boron nitride/graphene/boron nitride heterojunction preparation process
The preparation process of boron nitride/graphene/boron nitride heterojunction is mainly divided into three parts:
1) Transfer process of graphene and boron nitride;
2) Heterostructure synthesis process;
3) Manufacturing process of substrate and electrode.
2.1 Transfer process of graphene and boron nitride
2.1.1 Graphene transfer process
The graphene film prepared by chemical vapor deposition method uses metal Cu or Ni as the substrate (taking Cu as an example), in order to achieve film transfer, the Cu substrate needs to be removed first.Cu can be etched by strong acids such as strong sulfuric acid and strong nitric acid, but this process reacts violently, produces a large number of H2 bubbles, destroys the structure of graphene, so the Cu layer is not etched with strong acid. In addition to strong acids, Cu can also be dissolved in FeCl3 solution, and the specific chemical reaction formula is
The advantage of using FeCl3 solution to remove Cu layer is that there is no gaseous product or solid deposition, and the reaction is gentle and not violent. In the process of removing the Cu layer, the flexible polymer PMMA is spun coated on the surface of graphene to block external metal ions, dust and other pollution and prevent its electrical properties from changing. The specific transfer process is shown in Figure 3.
Figure 3 Graphene transfer process flow diagram
The first step of the transfer process is to spin apply the PMMA protective layer, fix the sample to the homogenization table, set the 1st speed to 700r/min for 9s, set the 2nd stage speed to 3000r/min for 40s,
3~5 drops of PMMA solution are dropped to the sample surface with a dropper, and the thickness of the PMMA layer after spinning coating should be between 1~1.5μm.
The sample was placed on a hot plate at 90°C for pre-drying film, the purpose was to solidify the liquid PMMA layer, reduce the standing wave effect, and the pre-drying time was 1min.
Drop 2~3 drops of PMMA solution to the sample surface, maintain the homogenization table speed during the first spin coating, and the thickness of the PMMA layer should be between 1.8~2.3μm after the spinning is completed.
After the second spinning of the sample, the sample was placed on a hot plate at 90 °C for post-drying film, the duration was 40min.After the post-drying was completed, the Cu etching solution was configured, of which the FeCl3 concentration was 5g/100mL, accounting for 40%, a small amount of HCl solution, accounting for 5%, the sample was placed in the etching solution with tweezers, and under the buoyancy of the PMMA layer, the sample was semi-immersed on the surface of the etching solution.
Ensure that the Cu layer is in contact with the etching solution, the etching time is about 6h, when the Cu layer is etched, the PMMA/graphene layer will continue to float on the surface of the solution, and the substrate will sink to the bottom of the solution.
At this time, the target substrate that needs to be transferred graphene is clamped with tweezers, and the PMMA/graphene layer is fished out of the solution with the target substrate to ensure that the graphene layer is in contact with the target substrate.
Then the PMMA/graphene layer was released into the prepared deionized water for rinsing, the rinsing time was 20min/time, a total of 3 rinses, in order to remove various impurities and metal ions and other pollutants on the graphene surface. After rinsing, the PMMA/graphene/target substrate is placed on a flat experimental bench, the nitrogen gun is used to face the sample vertically, and the sample is dried by a small air flow, the purpose is to use the power of the air flow to ensure the close fit between graphene and the target substrate, reduce the folds and cracks of the graphene film, and remove the moisture on the sample surface to prevent bubbles under the graphene in later steps.
After vertical drying, the sample was placed on an 80°C hot plate for 20min to remove all water and increase the van der Waals force between graphene and the target substrate. Finally, the PMMA/graphene/target substrate sample was gently put into acetone solution and stood for 10min, the PMMA protective layer on the surface of graphene was dissolved, the sample was taken out after soaking and then put into alcohol to remove the acetone solution attached to the sample surface, and the sample was taken out to the nitrogen cabinet to dry after cleaning to complete the transfer of graphene.
2.1.2 Boron nitride transfer process
The transfer process of boron nitride is similar to that of graphene, and the specific process is shown in Figure 4.
Figure 4 Boron nitride transfer process flow diagram
2.2 Heterogeneous bonding process
The boron nitride/graphene/boron nitride heterojunction is composed of three parts: boron nitride on the top layer, boron nitride on the bottom layer and graphene, the thickness of boron nitride and boron nitride on the top layer is about 13nm, and the nanofilms are stacked vertically and combined by van der Waals forces. The specific synthesis process is shown in Figure 5.
Figure 5: Process flow diagram of heterostructure synthesis
Firstly, the prepared boron nitride film is transferred from the copper-based substrate to the target substrate as the bottom boron nitride by transfer process, the surface of the film is treated by vertical meteorological drying method, the van der Waals force between the film and the substrate is strengthened by the high-temperature baking method (80 °C, 15min), and finally the bottom boron nitride is graphed by photolithography process and oxygen plasma etching process.
The etching parameters are: gas is O2, power is 100W, flow rate is 70mL/min, pressure is 13Pa, etching time is 5min.After the preparation of boron nitride at the bottom layer, it is transferred as a substrate to transfer graphene, and the surface of the film is treated by vertical meteorological drying method during the transfer process, and the van der Waals force between the film and the substrate is strengthened by the high-temperature baking method to improve the quality of graphene.
After using photolithography process and etching process graphic graphene, the top layer of boron nitride was transferred with graphene as the substrate, and the van der Waals force between the nanofilms was enhanced by high-temperature baking method after the transfer was completed, and the preparation of boron nitride/graphene/boron nitride heterojunction could be completed after the high-temperature baking was completed.
2.3 Manufacturing process of substrate and electrode
The manufacturing process of substrate and electrode belongs to the conventional MEMS processing technology. It is mainly divided into three parts: deposition dielectric layer, etching pressure chamber and deposition electrode.
2.3.1 Sedimentary medium layer
In this paper, silicon dioxide was used as the dielectric layer of boron nitride/graphene/boron nitride heterojunction, and the dielectric layer was used to isolate the contact between the nanofilm layer and the silicon substrate to prevent current leakage, with a thickness of 1.5μm.
2.3.2 Etch pressure chamber
The pressure chamber is a square groove with a size of 64μm×6μm and a depth of 650nm, located in the silica dielectric layer. The pressure chamber was made by reactive ion etching process (RIE), and the etching rate was 28nm/min. The etching time is 1392s.
2.3.3 Deposition electrodes
The electrodes of boron nitride/graphene/boron nitride heterojunction are divided into bottom electrodes and top electrodes. The bottom electrode is deposited on the surface of the silica dielectric layer by magnetron sputtering process, and Ti metal with a thickness of 10nm is first deposited as an adhesion layer, and then a Pt metal electrode with a thickness of 50nm is deposited on the Ti metal layer, so as to complete the production of the bottom electrode.
After completing the transfer of the bottom boron nitride and graphene, the top metal electrode was deposited on the surface of the graphene film. In order to prevent the destruction of the nanofilm layer that has been transferred to the target substrate, the top electrode is selected to deposit the 50nm thickness Au metal electrode to the surface of the graphene film by electron beam evaporation process.The size of PAD is 200μm×200μm, and the gold wire with a diameter of 20μm is welded to the PAD by pressure welding process to elicit the output signal of the mechanically sensitive unit.
3. Characterization and testing of boron nitride/graphene/boron nitride heterojunction
3.1 Boron nitride/graphene/boron nitride heterojunction samples
Boron nitride/graphene/boron nitride heterojunctions can be prepared by the above manufacturing process, and the device structure observed by scanning tunneling microscopy (SEM) is shown in Figure 6.
The finished sample is sliced as shown in Figure 6(a), which contains 54 boron nitride/graphene/boron nitride heterojunctions, with a boron nitride/graphene/boron nitride heterojunction between every two PADs. The magnified boron nitride/graphene/boron nitride heterojunction is shown in Figure 6(b) and (c). As shown in Figure 6(d), the boron nitride/graphene/boron nitride heterojunction was electrically tested by a probe stage.
Figure 6: Graphene mechanical sensitive element sample
3.2 Quality characterization of boron nitride/graphene/boron nitride heterojunction
The preparation quality of boron nitride/graphene/boron nitride heterojunction directly affects the performance of the device. Using Raman spectroscopy (HR-800, Horiba Scientific, Inc.) The preparation quality of boron nitride/graphene/boron nitride heterojunction was tested, and the test results are shown in Figure 7.
Figure 7 marks the position of the Raman spot in the test, the test results show that the G peak position is 1586cm-1, the 2D peak position is 2683cm-1, the ratio of 2D peak intensity I2D to G peak intensity IG is 5.34.Figure 7(a) is the Raman spectrum of the graphene force-sensitive device manufactured by the reference [6], it can be seen that the ratio of the Raman spectrum 2D peak intensity I2D to G peak intensity IG obtained in the literature is only about 1.2, which proves that the boron nitride / graphene / Boron nitride heterojunctions have very high quality.
Figure 7 Graphene Raman spectroscopy
In this paper, the preparation process of boron nitride/graphene/boron nitride heterojunction was studied, and a complete set of boron nitride/graphene/boron nitride heterojunction manufacturing process scheme was proposed, and trial production was carried out. In the Raman test results, the G peak position was 1586cm-1, the 2D peak position was 2683cm-1, and the ratio of the 2D peak intensity I2D to the G peak intensity IG was 5.34, which indicated that the prepared boron nitride/graphene/boron nitride heterojunction had good quality, and also proved that the preparation process of high-quality boron nitride/graphene/boron nitride heterojunction proposed in this paper was feasible, and provided a research basis for the fabrication technology of graphene devices based on boron nitride/graphene/boron nitride heterostructure.