Ferroelectric metasurface and preparation method thereof

Abstract

The invention relates to a ferroelectric metasurface and a preparation method, the ferroelectric metasurface is suitable for a terahertz wave band, the ferroelectric metasurface comprises a silicon substrate, a ferroelectric film layer and a metal resonance layer which are stacked in sequence, the ferroelectric film layer is a scandium nitride aluminum film, and the dielectric constant of the ferroelectric film layer is 14-20. Scattering resonance of terahertz waves can be achieved, and electromagnetic stealth of one frequency point in the terahertz wave band is achieved.

Description

TECHNICAL FIELD

[0001] The application relates to a metasurface modulator, in particular to a ferroelectric metasurface applied in a terahertz wave band and a preparation method thereof. BACKGROUND

[0002] Terahertz waves are a part of the electromagnetic spectrum between the infrared and microwave frequency bands, with a frequency range from 0.1 THz to 10 THz and a corresponding wavelength range from 30 microns to 3 mm. Terahertz waves have low photon energy, high temporal and spatial coherence, and can be used for sub-picosecond, femtosecond time-resolved transient spectroscopy, and have extremely important applications in security inspection, medical imaging, communication, etc. A terahertz metasurface modulator can control the amplitude and frequency of terahertz waves in a circuit, thereby effectively processing terahertz wave signals. At present, terahertz metasurface modulators are mainly realized by temperature control, light control and electric control. A terahertz temperature-controlled metasurface modulator can achieve a large modulation dynamic range, but its performance is limited by a low modulation speed. A terahertz light-controlled metasurface modulator can also achieve a large modulation dynamic range and has a relatively higher modulation speed than the temperature control modulation scheme, but it usually needs a high-power ultrashort pulse laser as an excitation light source, which has high power consumption and low integration, and is difficult to meet the actual application requirements. An electrically controlled terahertz metasurface modulator usually changes the carrier concentration in a semiconductor material by applying an external electric field, thereby affecting the response of the metasurface to terahertz waves. Compared with the light control and temperature control schemes, the electrically controlled terahertz metasurface modulator usually has the advantages of high modulation speed, low power consumption and high integration.

[0003] With the rapid development of electronic technology, ferroelectric materials have been widely used in the development of phase shift elements in phased array antennas and tuning elements in devices operating in the terahertz frequency band. Therefore, it is of great significance to develop an electrically controlled terahertz metasurface modulator based on ferroelectric materials for the development of terahertz devices. SUMMARY

[0004] The technical problem to be solved by the application is to provide a ferroelectric metasurface applied in a terahertz wave band and a preparation method thereof, which can realize scattering resonance of terahertz waves and electromagnetic invisibility of a frequency point in the terahertz wave band.

[0005] According to a first aspect of an embodiment of the application, a ferroelectric metasurface is provided, which is suitable for a terahertz wave band and comprises: a silicon substrate, a ferroelectric film layer and a metal resonance layer which are sequentially stacked, the ferroelectric film layer is a scandium aluminum nitride film, and the dielectric constant of the ferroelectric film layer is 14-20.

[0006] In some embodiments, the thickness of the ferroelectric film layer is 0.1-0.3 microns.

[0007] In some embodiments, the thickness of the silicon substrate is 0.4mm-0.6mm.

[0008] In some embodiments, the thickness of the metal resonance layer is 10μm-50μm.

[0009] In some embodiments, the metal resonance layer comprises a periodic array of patterned aluminum metal structures.

[0010] In some embodiments, the content of scandium in the ferroelectric thin film layer is 20%-30%.

[0011] In some embodiments, the aluminum metal structure comprises two horizontal arms and a vertical arm connecting the two horizontal arms, and the two horizontal arms are circular arc-shaped, straight line-shaped or curve-shaped.

[0012] According to a second aspect of the embodiments of the present application, a preparation method of a ferroelectric metasurface is provided, which comprises: providing a silicon substrate; depositing a ferroelectric thin film layer on the silicon substrate by a magnetron sputtering process, the ferroelectric thin film layer being a scandium aluminum nitride film with a dielectric constant of 14-20; forming a metal resonance layer on the ferroelectric thin film layer by a photolithography process.

[0013] In some embodiments, the depositing a ferroelectric thin film layer on the silicon substrate by a magnetron sputtering process comprises: depositing at a preset working pressure, controlling the radio frequency power of aluminum to be 70-185W, the radio frequency power of scandium to be 0-225W, and performing the deposition under 60SCCM pure nitrogen.

[0014] In some embodiments, the thickness ratio of the ferroelectric thin film layer and the silicon substrate is controlled to achieve electromagnetic invisibility at a preset frequency point, and the preset frequency point is any frequency point in a 0.1-1THz terahertz wave band.

[0015] Compared with the prior art, the present application has the beneficial effects that: by forming a scandium aluminum nitride film and a metal resonance layer on a silicon substrate, the dielectric constant of the scandium aluminum nitride film is 14-20, and the dielectric constants of silicon, scandium aluminum nitride and the metal resonance layer are linked to achieve scattering resonance of terahertz waves and electromagnetic invisibility at a frequency point in the terahertz wave band; the scandium aluminum nitride film has a relatively high dielectric constant and a relatively low loss in the terahertz wave band, and the ferroelectric polarization state can be maintained after the external electric field is removed, which enables the metasurface modulator to achieve non-volatile modulation and greatly reduces static power consumption, which is a major advantage over modulators based on carrier injection, phase change materials, liquid crystals and the like which require continuous power supply. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1is a schematic diagram of a ferroelectric metasurface applied in a terahertz wave band according to an exemplary embodiment.

[0017] Figure 2 is a schematic diagram of a metal resonant layer according to an exemplary embodiment.

[0018] Figures 3a-3c is a schematic diagram of a metal structure in a metal resonant layer according to an exemplary embodiment.

[0019] Figure 4 is a flow chart of preparation of a ferroelectric metasurface according to an exemplary embodiment.

[0020] Figure 5 is a graph of the relationship between the thickness of a ferroelectric film layer in a ferroelectric metasurface and electromagnetic invisibility at each frequency point in a terahertz wave band.

[0021] Figure 6 is a graph of the relationship between the thickness of a substrate in a ferroelectric metasurface and electromagnetic invisibility at each frequency point in a terahertz wave band. DETAILED DESCRIPTION

[0022] Unless otherwise defined, technical or scientific terms used in this specification and claims shall have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. In the following description, specific embodiments of the application are described in connection with the appended drawings, in which it is noted that, for purposes of simplicity and clarity, the description is not possible to describe all the features of the actual implementation. In the spirit and scope of the application, modifications and replacements of the embodiments of the application can be made by those skilled in the art, and the resulting embodiments are also within the scope of protection of the application.

[0023] A metasurface is an ultrathin artificial surface composed of microstructure units, which can flexibly control the amplitude, phase and polarization of electromagnetic waves. The excellent performance of metasurfaces in phase gradient and negative refraction makes them show great potential in electromagnetic wave control. A large number of researchers combine materials with tunable properties and semiconductor materials to form tunable metasurface devices, which realize dynamic control of terahertz waves under external excitations such as electric field, light and temperature.

[0024] The application provides a ferroelectric metasurface applied in a terahertz wave band, referring to Figure 1In a specific embodiment, it comprises: a silicon substrate 13, a ferroelectric thin film layer 12 and a metal resonance layer 12 stacked in sequence, the ferroelectric thin film layer is a scandium aluminum nitride (AlScN) thin film, and the dielectric constant of the ferroelectric thin film layer 12 is 14-20. The present application uses a scandium aluminum nitride thin film with a dielectric constant of 14-20 for the ferroelectric thin film layer 12, and links silicon, AlScN and the metal in the metal resonance layer, to achieve scattering resonance of terahertz waves (0.1-1 THz), so as to realize electromagnetic invisibility at any frequency point in the terahertz wave band. The scandium aluminum nitride thin film has relatively low loss in the terahertz wave band, so it can be applied to the terahertz wave band. The scandium aluminum nitride thin film can change its polarization direction after applying a voltage, so the present application can realize dynamic terahertz wave modulation. Since its electrically controlled modulation does not depend on high-loss carriers or material phase change (such as the metallic state of VO2), the insertion loss of the super surface of the present application can be made lower.

[0025] The present application uses a metal resonance layer 11 as an artificial atom, and excites local super surface electromagnetic resonance through its geometric shape. The silicon substrate 13 provides a high refractive index environment to enhance the square localization; the ferroelectric thin film layer 12, i.e. the scandium aluminum nitride thin film, as a tunable dielectric layer, its dielectric constant ε fe Under the action of an external bias voltage V bias control. When the terahertz wave is incident as an incident wave to the metal resonance layer 11, an electric dipole or magnetic dipole resonance is excited thereby; an external voltage is used to change the dielectric constant of the ferroelectric thin film layer 12 to realize resonance frequency control, and at the resonance frequency, the super surface of the present application has a significantly enhanced scattering cross section as a modulator for the incident wave, realizing electromagnetic invisibility at the resonance frequency. The super surface of the present application as a modulator can be used for amplitude and phase modulation of terahertz waves.

[0026] In some embodiments, the thickness of the above-mentioned ferroelectric thin film layer 12, i.e. the AlScN thin film, is 0.1-0.3 μm. It has been found through research and simulation analysis certification by the CST Studio electromagnetic simulation module that the frequency point of electromagnetic invisibility is related to the thickness of the AlScN thin film, as shown in Figure 5 different thicknesses of the AlScN thin film correspond to electromagnetic invisibility at different frequency points, based on which the thickness of the AlScN thin film can be set according to the frequency point to be hidden. For example, if the thickness of the AlScN thin film is 200 nm, it corresponds to electromagnetic invisibility at 0.7 THz; if the thickness of the AlScN thin film is 100 nm, it corresponds to electromagnetic invisibility at 0.62 THz; and if the thickness of the AlScN thin film is 200 nm, it corresponds to electromagnetic invisibility at 0.65 THz.

[0027] Specifically, the content of scandium in the ferroelectric thin film layer 12, i.e., the AlScN thin film, is 20% to 30%, and the dielectric constant of the ferroelectric thin film layer 12 increases with the content of scandium. The AlScN thin film destroys the hexagonal symmetry of AlN by doping Sc, and induces the thin film to be ferroelectric.

[0028] In some embodiments, the thickness of the silicon substrate 13 is 0.4 mm to 0.6 mm. Simulation analysis by CST Studio electromagnetic simulation module shows that the frequency point of electromagnetic invisibility is related to the thickness of the silicon substrate 13, as shown in Figure 6 The silicon substrate 13 with different thicknesses corresponds to electromagnetic invisibility at different frequency points. Based on this, in combination with the influence of the thickness of the AlScN thin film on the frequency point of electromagnetic invisibility in Figure 5 , in an embodiment, the thickness of the silicon substrate 13 is 500 μm, and the thickness of the AlScN thin film is 200 nm, which can achieve electromagnetic invisibility of 0.7 THz.

[0029] In some embodiments, the thickness of the metal resonant layer is 10 μm to 50 μm. In an embodiment, as shown in Figure 2 The metal resonant layer 11 includes a periodic array of patterned aluminum metal structures. As shown in Figures 3a-3c , the shape of the aluminum metal structure can be any geometric shape, such as a C-shaped open ring, a cross, a work shape, an H shape, a butterfly shape, etc. Alternatively, the aluminum metal structure includes two horizontal arms and a vertical arm connecting the two horizontal arms, and the two horizontal arms are circular arc-shaped, linear or curved. The shape of the aluminum metal structure needs to be coupled with the AlScN thin film. Alternatively, as shown in Figure 2 , the aluminum metal structure of the work shape is selected to achieve scattering resonance of terahertz waves (0.1-1 THz) and achieve electromagnetic invisibility of 0.7 THz.

[0030] An embodiment of the present application provides a preparation method of a ferroelectric metasurface, which is applied to electromagnetic invisibility in the terahertz wave band, and the specific structure can be the ferroelectric metasurface disclosed in any of the above embodiments. The preparation method of the present embodiment, as shown in Figure 4 , includes the following steps: providing a silicon substrate; depositing a ferroelectric thin film layer on the silicon substrate by a magnetron sputtering process, the ferroelectric thin film layer being a scandium aluminum nitride thin film with a dielectric constant of 14 to 20; forming a metal resonant layer on the ferroelectric thin film layer by a photolithography process.

[0031] In an embodiment, the silicon substrate is a low-resistance (0.001-0.005 Ωcm) n-type (111) Si substrate, which can be pretreated to avoid interface stress when forming the ferroelectric thin film layer. Specifically, the silicon substrate can be immersed in hydrofluoric acid (5 vol%) to remove the natural oxide layer.

[0032] The ferroelectric thin film layer is deposited on the silicon substrate using a magnetron sputtering process, specifically including: At a preset operating pressure, typically 0.52 Pa, the RF power for aluminum can be controlled to be 70-185 W, fixed at 185 W in this embodiment. The power range for scandium is 0-225 W, and deposition is performed under 60 SCCM of pure nitrogen. The use of pure nitrogen can avoid the formation of nitrogen defects during the doping process.

[0033] To ensure ferroelectric properties, it is necessary to ensure that the scandium content is uniform. In one embodiment, an aluminum-scandium alloy is used as the target material to ensure a constant scandium doping amount.

[0034] To achieve optimal deposition, in one embodiment, the silicon substrate is heated to 200°C to 400°C, optionally 350°C, and the deposition is performed at a rotational speed of 5 rpm to ensure uniformity in the composition and thickness of the ferroelectric film. This embodiment utilizes a low-temperature magnetron sputtering process, which improves compatibility with CMOS fabrication processes and facilitates production.

[0035] In some embodiments, the thickness ratio of the ferroelectric thin film layer to the silicon substrate is controlled. Specifically, the thickness of the ferroelectric thin film layer and the thickness of the silicon substrate can be controlled separately to achieve electromagnetic stealth at a preset frequency, which is any frequency within the 0.1-1 THz terahertz band. Figure 5 and Figure 6 As shown, the thickness of the silicon substrate 13 is 500 μm, and the thickness of the AlScN film is 200 nm, which can achieve electromagnetic stealth of 0.7 THz.

[0036] Specifically, according to Figure 5 Obtaining the thickness of the ferroelectric thin film layer used at different electromagnetic stealth frequencies, obtaining a plurality of first data points of the electromagnetic stealth frequencies and the thickness of the ferroelectric thin film layer, and fitting the plurality of first data points into a first curve; Figure 6 The thickness of the silicon substrate used for different electromagnetic stealth frequencies is obtained, and second data points of multiple electromagnetic stealth frequencies and silicon substrate thicknesses are obtained, and the multiple second data points are fitted into a second curve; the first curve and the second curve are used as standard curves. When preparing a metasurface with electromagnetic stealth at a preset frequency, the thickness of the ferroelectric thin film layer and the thickness of the silicon substrate can be selected according to the standard curve, thereby realizing parameter setting of the magnetron sputtering process and the silicon substrate back polishing process.

[0037] In a specific embodiment, a method for preparing a ferroelectric metasurface includes: Providing a low-resistance n-type (111) Si substrate, pre-treating the Si substrate, and removing a surface oxide layer; AlScN thin films were deposited on Si substrates using a magnetron sputtering process. The thickness of the AlScN film was 200 nm, the scandium content was 20%, and the dielectric constant of the AlScN film was 14. deposited on the AlScN film is 50 μm thick; The aluminum metal layer is patterned using a photolithography process to form a periodic array of metal structures as shown in FIG. 2B. Figure 2 The aluminum metal layer is patterned using a photolithography process to form a periodic array of metal structures as shown in FIG. 2B. The ferroelectric metasurface shown in FIG. 2C can be subsequently processed through annealing, backside polishing of the silicon substrate, etc. Figure 1 The ferroelectric metasurface shown in FIG. 2C can be subsequently processed through annealing, backside polishing of the silicon substrate, etc.

[0038] In another embodiment, a method for preparing a ferroelectric metasurface includes: A low-resistance n-type (111) Si substrate is provided, and the Si substrate is pretreated to remove the oxide layer on the surface; An AlScN film is deposited on the Si substrate using a magnetron sputtering process, the AlScN film has a thickness of 150 nm, the scandium content is 24%, and the dielectric constant of the AlScN film is 16; A 50 μm thick aluminum metal layer is deposited on the AlScN film; The aluminum metal layer is patterned using a photolithography process to form a periodic array of metal structures as shown in FIG. 2B. Figure 3a The aluminum metal layer is patterned using a photolithography process to form a periodic array of metal structures as shown in FIG. 2B. The ferroelectric metasurface shown in FIG. 2C can be subsequently processed through annealing, backside polishing of the silicon substrate, etc. Figure 1 The ferroelectric metasurface shown in FIG. 2C can be subsequently processed through annealing, backside polishing of the silicon substrate, etc.

[0039] In another embodiment, a method for preparing a ferroelectric metasurface includes: A low-resistance n-type (111) Si substrate is provided, and the Si substrate is pretreated to remove the oxide layer on the surface; An AlScN film is deposited on the Si substrate using a magnetron sputtering process, the AlScN film has a thickness of 100 nm, the scandium content is 27%, and the dielectric constant of the AlScN film is 18; A 40 μm thick aluminum metal layer is deposited on the AlScN film; The aluminum metal layer is patterned using a photolithography process to form a periodic array of metal structures as shown in FIG. 2B. Figure 3a The aluminum metal layer is patterned using a photolithography process to form a periodic array of metal structures as shown in FIG. 2B. The ferroelectric metasurface shown in FIG. 2C can be subsequently processed through annealing, backside polishing of the silicon substrate, etc. Figure 1 The ferroelectric metasurface shown in FIG. 2C can be subsequently processed through annealing, backside polishing of the silicon substrate, etc.

[0040] The above description of the embodiments is given for the purpose of completeness to provide one of ordinary skill in the art with a thorough understanding of the application and does not limit the application to any one embodiment or application. It will be apparent to those skilled in the art that various modifications and variations can be made to the present embodiments without departing from the scope or spirit of the application. Thus, it is intended that the present application cover modifications and variations of this application provided they come within the scope of the appended claims and their equivalents.

Claims

1. A ferroelectric metasurface suitable for terahertz band, comprising: A silicon substrate, a ferroelectric thin film layer and a metal resonance layer are stacked in sequence, wherein the ferroelectric thin film layer is a scandium aluminum nitride thin film, and the dielectric constant of the ferroelectric thin film layer is 14-20.

2. The ferroelectric metasurface according to claim 1, wherein The thickness of the ferroelectric thin film layer is 0.1 μm to 0.3 μm.

3. The ferroelectric metasurface according to claim 1, wherein The thickness of the silicon substrate is 0.4 mm to 0.6 mm.

4. The ferroelectric metasurface according to any one of claims 1 to 3, wherein The thickness of the metal resonance layer is 10 μm to 50 μm.

5. The ferroelectric metasurface according to any one of claims 1 to 3, wherein The metal resonance layer includes a periodic array of patterned aluminum metal structures.

6. The ferroelectric metasurface according to claim 5, wherein: The scandium content in the ferroelectric thin film layer is 20% to 30%.

7. The ferroelectric metasurface according to claim 6, wherein: The aluminum metal structure includes two horizontal arms and a vertical arm connecting the two horizontal arms, and the two horizontal arms are both in an arc shape, a straight line shape or a curve shape.

8. A method for preparing a ferroelectric metasurface, characterized in that: include: providing a silicon substrate; A ferroelectric thin film layer is deposited on the silicon substrate by a magnetron sputtering process, wherein the ferroelectric thin film layer is a scandium aluminum nitride thin film with a dielectric constant of 14 to 20; A metal resonance layer is formed on the ferroelectric thin film layer by adopting a photolithography process.

9. The preparation method according to claim 8, wherein The method of depositing a ferroelectric thin film layer on the silicon substrate using a magnetron sputtering process comprises: At a preset operating pressure, the RF power for aluminum was controlled between 70 and 185 W, and the RF power for scandium was controlled between 0 and 225 W. Deposition was performed under 60 SCCM of pure nitrogen.

10. The preparation method according to claim 8, characterized in that The thickness ratio of the ferroelectric thin film layer to the silicon substrate is controlled to achieve electromagnetic stealth at a preset frequency point, where the preset frequency point is any frequency point within the terahertz band of 0.1 to 1 THz.

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