PZT-based Graphene Composite Piezoresistive Film Structure Based on the Enhancement of the Force-Electric Coupling Mechanism by Flexoelectric Polarization

Abstract

The present invention belongs to the field of NEMS micro-pressure sensitive sensor design, specifically a PZT-based graphene composite pressure-sensitive thin film structure based on the flexoelectric polarization-enhanced force-electric coupling mechanism, which includes a Si / SiO2 substrate. A back cavity is provided on the Si / SiO2 substrate. A metal layer of pt / Ti is sputtered on the front surface of the Si / SiO2 substrate. A PZT ferroelectric thin film is sputtered on the metal layer of pt / Ti. A graphene thin film is on the PZT ferroelectric thin film. Metal electrodes are sputtered on both sides of the graphene thin film. A packaging substrate is on the back surface of the Si / SiO2 substrate. The present invention regulates the graphene thin film through the polarization voltage generated by the bending of the PZT ferroelectric thin film, applies a bias voltage to the graphene thin film, indirectly characterizes the transport of graphene carriers through the output current of the circuit, and further characterizes the change in the energy band of the graphene thin film, completing the exploration of the mechanism of regulating the energy band structure and electrical transport of graphene by PZT flexoelectric polarization, and completing the design of a high-sensitivity force-electric coupling PZT-based graphene composite pressure-sensitive thin film structure.

Description

Technical Field

[0001] The present invention belongs to the field of NEMS micro-pressure sensitive sensor design, and specifically relates to a PZT-based graphene composite piezoresistive thin film structure based on the flexoelectric polarization enhanced force-electric coupling mechanism. Background Art

[0002] The flexoelectric effect has become increasingly prominent in the research of two-dimensional materials at the nanoscale. S. Park et al. from Seoul National University, South Korea, used a PFM microscope to apply mechanical pressure to a BiFeO3 ferroelectric thin film. The results showed that the polarization of the ferroelectric domains inside the ferroelectric thin film flipped under the strain gradient, and the out-of-plane electric field distribution changed. Phase field simulation indicated that the flexoelectric field inside the material played a crucial role in this polarization regulation. D. Lee et al. from Seoul National University prepared a composite interface structure of a HoMnO3 thin film substrate with a thickness of 10 nm by molecular beam epitaxy. Its strain gradient was increased by 6-7 orders of magnitude compared with traditional bulk materials, and the internal flexoelectric field was also significantly enhanced.

[0003] Based on the flexoelectric enhancement effect of ferroelectric thin films at the micro-nano scale, the electrical properties of surface two-dimensional materials can be regulated by using their surface polarization potential and charge accumulation. A. Gruverman from the University of Nebraska prepared a LaAlO3 / SrTiO3 two-dimensional thin film composite cantilever structure. By applying an external force to this composite cantilever structure using a PFM microscope, due to the flexoelectric polarization effect, an interfacial polarization electrode and accumulated charges will be generated at the interface of the two-dimensional thin film material, changing the output values of the high-resistance state and low-resistance state of the conductivity under a bias voltage, and realizing the force-electric response regulation of the two-dimensional thin film material based on the flexoelectric effect. Professor Hu Zhigao from East China Normal University systematically observed the flexoelectric regulation of optoelectronic response characteristics of n-type (InSe) and p-type (WSe2) two-dimensional semiconductors experimentally through a piezoresponse force microscope and a photo-excited Kelvin probe microscope.

[0004] In view of the significant improvement of the flexoelectric effect of ferroelectric materials at the micro-nano scale, scholars at home and abroad have designed and manufactured various ferroelectric micro-nano structures. It is a very meaningful work to regulate the electrical response of thin film materials on the surface of devices, especially two-dimensional thin film materials, by means of the flexoelectric enhanced polarization potential and accumulated charges of the micro-nano structure. Summary of the Invention

[0005] The present invention proposes a PZT-based graphene composite piezoresistive thin film structure based on the flexoelectric polarization-enhanced force-electric coupling mechanism, and constructs a force-electric coupling simulation model based on the flexoelectric effect on the PZT ferroelectric thin film. The simulation model is established using COMSOL Multiphysics software. The flexoelectric effect is characterized by the strain gradient of the PZT ferroelectric thin film, and the size effect of the flexoelectric effect at the micro-nano scale is studied. The variation law of the output voltage of the force-electric coupling effect of the PZT ferroelectric thin film based on the flexoelectric effect at the micro-nano scale is obtained. And a method of regulating the energy band of the graphene thin film by the polarization voltage of the PZT ferroelectric thin film to change its carrier transport situation is proposed. Finally, based on these studies, a PZT-based graphene composite piezoresistive thin film structure based on the flexoelectric polarization-enhanced force-electric coupling mechanism is designed.

[0006] The present invention is implemented by the following technical solutions: The PZT-based graphene composite piezoresistive thin film structure based on the flexoelectric polarization-enhanced force-electric coupling mechanism includes a Si / SiO2 substrate. There is a back cavity on the Si / SiO2 substrate. A metal layer of pt / Ti is sputtered on the front of the Si / SiO2 substrate. A PZT ferroelectric thin film is sputtered on the metal layer of pt / Ti. There is a graphene thin film on the PZT ferroelectric thin film. Metal electrodes are sputtered on both sides of the graphene thin film. There is a packaging substrate on the back of the Si / SiO2 substrate.

[0007] The preparation process includes the following steps:

[0008] 1) Cut and clean the Si / SiO2 substrate to obtain a silicon substrate with both shape and cleanliness meeting the requirements;

[0009] 2) Sputter a metal layer of pt / Ti on the Si / SiO2 substrate;

[0010] 3) On the basis of step 2, sputter a PZT ferroelectric thin film with a certain thickness;

[0011] 4) Etch a back cavity on the Si / SiO2 substrate from the back;

[0012] 5) Then transfer the graphene thin film onto the PZT ferroelectric thin film;

[0013] 6) Sputter metal electrodes with set sizes on both sides of the graphene thin film;

[0014] 7) Perform back cavity packaging on the substrate.

[0015] The specific steps of the design method are as follows:

[0016] (1)Establish a force-electric coupling simulation model of PZT ferroelectric thin films based on the flexoelectric effect: Establish a force-electric coupling simulation model of nano-ferroelectric thin films, and use COMSOL Multiphysics software to simulate the force-electric coupling effect at the micron scale;

[0017] (2)Explore the flexoelectric effect and the size effect of force-electric coupling: On the basis of step (1), parametric scanning settings are made for the thickness of the PZT ferroelectric thin film, so as to calculate and simulate the force-electric response of PZT ferroelectric thin films with different thicknesses; The results show that at the micro-nano scale, the force-electric coupling effect gradually increases with the decrease of size;

[0018] (3)Explore the flexoelectric enhancement mechanism of strain gradient-induced polarization at the micro-nano scale: Through the analysis of the variation trends of strain gradient and strain piezoelectric effect with the thickness of nano-PZT ferroelectric thin films, on the basis of step (2), explore the variation law of the force-electric coupling effect at the micro-nano scale, and analyze the role of flexoelectricity of strain gradient-induced polarization therein;

[0019] (4)Explore the variation law of the output voltage of the force-electric coupling effect: Simulate the output electric potential of the PZT ferroelectric thin film under different pressure conditions, and draw a force-electric output relationship diagram to provide necessary data support for the flexoelectric polarization regulation of the energy band structure and electric transport mechanism of graphene by ferroelectric thin films;

[0020] (5)Explore the mechanism of polarization voltage regulation of the energy band of graphene thin film by PZT ferroelectric thin film: On the basis of the above simulation results, construct a PZT / CVD graphene composite structure, and regulate the graphene thin film through the polarization voltage generated by the bending of the PZT ferroelectric thin film, and then complete the exploration of the mechanism of PZT flexoelectric polarization regulation of the energy band structure and electric transport mechanism of graphene;

[0021] (6)Design the structure of a high-sensitivity force-electric coupling sensor device: Complete the design of the structure of a high-sensitivity force-electric coupling PZT-based graphene composite piezoresistive thin film, and formulate the preparation process of the PZT-based graphene composite piezoresistive thin film structure, so as to make theoretical preparations for the preparation of corresponding devices.

[0022] Regulate the graphene thin film through the polarization voltage generated by the bending of the PZT ferroelectric thin film, apply a bias voltage to the graphene thin film, indirectly characterize the transport of graphene carriers through the output current of the circuit, and then characterize the change of the energy band of the graphene thin film, complete the exploration of the mechanism of PZT flexoelectric polarization regulation of the energy band structure and electric transport mechanism of graphene, and complete the design of the structure of a high-sensitivity force-electric coupling PZT-based graphene composite piezoresistive thin film. Description of the Drawings

[0023] Figure 1 It is a schematic diagram of the device structure of the present invention.

[0024] Figure 2 It is a simulation diagram of stress gradient changes at different scales.

[0025] Figure 3 It is a distribution diagram of surface potential changes at different thicknesses.

[0026] Figure 4 It is a schematic diagram of ferroelectric-based external electric field regulating graphene carrier transport.

[0027] Figure 5 It is a process flow diagram of a PZT-based graphene composite micro-pressure sensor.

[0028] In the figure: 1 - Si, 2 - SiO2, 3 - metal layer of pt / Ti, 4 - PZT ferroelectric thin film, 5 - graphene thin film, 6 - metal electrode. Specific implementation mode

[0029] The present invention proposes a PZT-based graphene composite piezoresistive thin film structure based on flexoelectric polarization-enhanced force-electric coupling mechanism, and constructs a force-electric coupling simulation model based on the flexoelectric effect on the PZT thin film. By constructing the PZT-based force-electric coupling simulation model, the flexure degree changes and interface potential / charge evolution laws caused by external forces on the PZT ferroelectric thin film at the nanoscale are systematically studied. Combining the potential evolution law, a PZT-based graphene micro-pressure sensitive sensing structure is designed to explore the regulation mechanism of the flexoelectric polarization interface on the energy band structure and carrier transport of the attached graphene, providing theoretical and technical support for the expansion of applications of flexoelectric electronics in high-sensitivity testing in the nanoscale field.

[0030] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0031] 1. The specific steps of the design method of the PZT-based graphene flexoelectric polarization-enhanced high-sensitivity micro-pressure sensor are as follows:

[0032] (1) Establish a force-electric coupling simulation model of the PZT ferroelectric thin film based on the flexoelectric effect

[0033] Establish a force-electric coupling simulation model of the nano PZT ferroelectric thin film as Figure 2, a simulation model was created using the model wizard of COMSOL Multiphysics software. The two-dimensional module was selected, and then the steady-state analysis of the piezoelectric module with built-in multi-physics coupling in the software was chosen for research. A rectangular PZT ferroelectric thin film with a size of 1μm×50nm was drawn. Fixed constraints were applied to the left and right surfaces, the lower surface was grounded, and a uniform load of 1 mbar was applied to the upper surface. Finally, the simulation was carried out and the strain voltage was output.

[0034] (2) Explore the flexoelectric effect and the size effect of force-electric coupling

[0035] Based on (1), parametric scanning settings were made for the thickness of the PZT ferroelectric thin film ( Figure 3 ), and simulations were carried out from 50nm to 10μm with a step of 500nm. The strain voltage and thickness of the PZT ferroelectric thin film with different thicknesses were plotted in a two-dimensional coordinate system, as shown in Figure 3 . By analyzing the coordinate curve, it can be seen that the force-electric coupling effect is significantly enhanced at the nanoscale.

[0036] (3) Explore the flexoelectric enhancement mechanism of strain gradient-induced polarization at the nanoscale

[0037] Explore the relationship between the flexoelectric strain gradient and the electrode polarization intensity of the PZT ferroelectric thin film. Based on (2), explore the mechanism of the flexoelectric effect enhancing the force-electric coupling effect at the micro-nano scale. At the micro-nano scale, the enhancement of the force-electric coupling effect by the flexoelectric effect is very obvious. The strain gradient is characterized by the deformation gradient in the Z-axis direction in the COMSOL Multiphysics simulation. The strain gradient increases significantly with the decrease of the thickness, that is, the flexoelectric effect increases with the decrease of the size. From the change trend of the strain voltage with the thickness, it can be seen that the change trend of the strain voltage is basically the same as that of the strain gradient. Combining with the size effect of the force-electric coupling effect, it can be seen that the enhancement effect of the flexoelectric on the force-electric coupling effect is very obvious at the micro-nano scale.

[0038] (4) Explore the variation law of the output voltage of the force-electric coupling effect

[0039] The output potential of the PZT ferroelectric thin film under different pressure conditions was simulated. And a two-dimensional coordinate diagram of the force-electric output was drawn. It provides necessary data support for the flexoelectric polarization regulation of the energy band structure and the electrical transport mechanism of graphene by the PZT ferroelectric thin film. Among them, the pressure change range is 1 mbar - 100 mbar, and the step is 5 mbar.

[0040] (5) Explore the mechanism of the polarization voltage of the PZT thin film regulating the energy band of the graphene thin film

[0041] Based on the above simulation results, a PZT / CVD graphene composite structure is constructed. The graphene film is regulated by the polarization voltage generated by the bending of the PZT ferroelectric film, and then the exploration of the PZT flexoelectric polarization regulation of the graphene energy band structure and the electrical transport mechanism can be completed.

[0042] Analyze the influence relationships of the conduction band, valence band, Fermi level, etc. of graphene on the carrier transport and conductivity of graphene;

[0043]

[0044]

[0045] In the formula, E f is the Fermi level, n represents the number of free charges per unit volume, I is the current, q is the charge of the free charge, s is the cross-sectional area of the conductor, and v is the rate of the directional movement of the free charge.

[0046] Study the polarization potential of the PZT ferroelectric film on the graphene energy band structure and the electrical transport regulation mechanism. The graphene film is regulated by the polarization voltage generated by the bending of the PZT ferroelectric film. A bias voltage is applied to the graphene film, and the transport situation of the graphene carriers is indirectly characterized by the output current of the circuit. Furthermore, the change of the energy band of the graphene film is characterized. Complete the exploration of the PZT flexoelectric polarization regulation of the graphene energy band structure and the electrical transport mechanism.

[0047] (6) Design the structure of a high-sensitivity force-electric coupling sensor

[0048] Complete the design of the structure of the high-sensitivity force-electric coupling sensor. Preliminary formulate the process flow chart for the preparation of the PZT / graphene micro-pressure sensor, and make theoretical preparations for the preparation of the corresponding device.

[0049] The process for the preparation of the PZT / graphene micro-pressure sensor includes the following steps:

[0050] 1) Cut and clean the Si / SiO2 substrate to obtain a silicon substrate with both the shape and cleanliness meeting the requirements;

[0051] 2) Sputter a metal layer 3 of pt / Ti with a required thickness on the Si / SiO2 substrate;

[0052] 3) On the basis of step 2, sputter a PZT ferroelectric film 4 with a certain thickness;

[0053] 4) Combine technologies such as photolithography and deep silicon etching to etch a micron-sized back cavity on the Si / SiO2 substrate from the back, and etch through the silicon substrate;

[0054] 5) After etching the back cavity, transfer the graphene film 5 on the front of the Si / SiO2 substrate;

[0055] 6) Sputter metal electrodes 6 of a set size on both sides of the graphene film 5;

[0056] 7) Finally, perform back cavity encapsulation on the substrate with glass, and the front view of the device is shown in Figure e.

Claims

1. A PZT-based graphene composite piezoresistive thin film structure based on a flexoelectric polarization enhanced force-electric coupling mechanism, characterized in that: It includes a Si / SiO2 substrate, with a back cavity provided on the Si / SiO2 substrate. A metal layer (3) of pt / Ti is sputtered on the front side of the Si / SiO2 substrate. A PZT ferroelectric thin film (4) is sputtered on the metal layer of pt / Ti. There is a graphene thin film (5) on the PZT ferroelectric thin film. Metal electrodes (6) are sputtered on both sides of the graphene thin film (5). There is a packaging substrate on the back side of the Si / SiO2 substrate. The specific steps of its design method are as follows: (1) Establish a force-electric coupling simulation model of the PZT ferroelectric thin film based on the flexoelectric effect: Establish a force-electric coupling simulation model of the nano-ferroelectric thin film, and use COMSOL Multiphysics software to simulate the force-electric coupling effect at the micron scale; (2) Explore the flexoelectric effect and the size effect of force-electric coupling: On the basis of step (1), parametric scanning settings are made for the thickness of the PZT ferroelectric thin film (4), so as to calculate and simulate the force-electric response of the PZT ferroelectric thin film (4) with different thicknesses; The results show that at the micro-nano scale, the force-electric coupling effect gradually increases with the decrease of the size; (3) Explore the flexoelectric enhancement mechanism of strain gradient-induced polarization at the micro-nano scale: Through the analysis of the change trends of the strain gradient and the strain piezoelectric effect with the thickness of the nano-PZT ferroelectric thin film, on the basis of step (2), explore the change law of the force-electric coupling effect at the micro-nano scale, and analyze the role of the flexoelectric of strain gradient-induced polarization in it; (4) Explore the change law of the output voltage of the force-electric coupling effect: Simulate the output electric potential of the PZT ferroelectric thin film (4) under different pressure conditions, and draw a force-electric output relationship diagram, providing necessary data support for the flexoelectric polarization regulation of the graphene energy band structure and the electric transport mechanism of the ferroelectric thin film; (5) Explore the mechanism of the polarization voltage of the PZT ferroelectric thin film regulating the energy band of the graphene thin film: On the basis of the above simulation results, construct a PZT / CVD graphene composite structure, and regulate the graphene thin film (5) through the polarization voltage generated by the bending of the PZT ferroelectric thin film, and then the exploration of the PZT flexoelectric polarization regulation of the graphene energy band structure and the electric transport mechanism can be completed; (6) Design the structure of a high-sensitivity force-electric coupling sensor: Complete the design of the structure of a high-sensitivity force-electric coupling PZT-based graphene composite pressure-sensitive thin film, and formulate the preparation process of the PZT-based graphene composite pressure-sensitive thin film structure, making theoretical preparations for the preparation of corresponding devices.

2. The PZT-based graphene composite piezoresistive thin film structure based on the flexoelectric polarization enhanced force-electric coupling mechanism according to claim 1, characterized in that: The preparation process includes the following steps: 1) Cut and clean the Si / SiO2 substrate to obtain a silicon substrate with both the shape and cleanliness meeting the requirements; 2) Sputter a metal layer (3) of pt / Ti on the Si / SiO2 substrate; 3) On the basis of step 2, sputter a PZT ferroelectric thin film (4) with a certain thickness; 4) Etch a back cavity on the Si / SiO2 substrate from the back side; 5) Then transfer the graphene thin film (5) on the PZT ferroelectric thin film (4); 6) Sputter metal electrodes (6) with set sizes on both sides of the graphene thin film (5); 7) Perform back cavity packaging on the substrate.

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