Perfect absorber and preparation method thereof

Abstract

The present invention discloses a perfect absorber, comprising a cylindrical array and a metal film layer located on one side of the cylindrical array. The present invention also discloses a method for preparing the perfect absorber. The perfect absorber of the present invention has no polarization-sensitive defects, realizes asymmetric absorption of the terahertz band and the infrared band, and has an extremely thin metal loss layer, small heat capacity and thermal conductivity, and high sensitivity, and has a wide range of applications in single-photon detection systems.

Description

Technical Field

[0001] The invention belongs to the technical field of electromagnetic wave absorption structures, and in particular relates to a perfect absorber and a preparation method thereof. Background Art

[0002] An electromagnetic wave absorption structure based on artificial synthetic materials, whose electromagnetic wave parameters and the electromagnetic parameters of the surrounding environment can achieve impedance matching, and the absorption rate in a specific band is 100%, so people call this electromagnetic wave absorption structure a perfect absorber. The development of absorbers began in World War II. In the last century, many absorber design methods were proposed, the most famous of which was the circuit simulation absorber. After entering the 21st century, metamaterial absorbers were first proposed in 2002; the first microwave material absorber was born in 2008. At the same time, a large number of metamaterial absorbers with different characteristics and structures have emerged. According to the bandwidth method, they can be divided into narrowband absorbers and broadband absorbers; according to the stacking method, they can be divided into single-layer planar structures and stacked structures, etc. The geometric shapes of metamaterial absorbers are often symmetrical unit structures. There are also many choices for the materials of metamaterial absorbers, which can basically be divided into the following four categories: precious metals or high-conductivity metals, such as gold, copper, aluminum, etc.; transition metals, such as chromium, nickel, titanium, etc.; conductive metal oxides and nitrides, such as titanium nitride; semiconductors or semimetals, such as silicon, gallium arsenide, etc.; superconducting materials, such as niobium, niobium nitride, etc. Similarly, the dielectric materials of metamaterial absorbers at different working wavelengths are also very different. Microwaves and millimeter waves often use materials such as glass and Teflon; terahertz bands use more materials such as air and polyimide; infrared bands use silicon dioxide and aluminum oxide; and visible and ultraviolet bands use silicon dioxide, titanium dioxide, magnesium fluoride, etc.

[0003] However, in order to achieve high absorption rate in the target frequency band, the existing perfect absorber often adopts the method of adding back plate gold or thicker metal layer. Although this method can achieve the goal of perfect absorption, due to the existence of the bottom metal layer, part or even most of the absorbed energy is completed by this metal layer. As a result, on the one hand, the absorber will block electromagnetic wave signals in other bands, and on the other hand, the energy absorbed by the bottom metal cannot be used for sensing, detection or imaging applications, thereby reducing the utilization efficiency of the electromagnetic field in the perfect absorber. At the same time, the thickness of the perfect absorber structure currently working in the millimeter wave, terahertz wave and infrared visible light bands is generally a few tenths of the wavelength, and it is rare to achieve one thousandth or even one ten-thousandth of the wavelength. When the perfect absorber becomes extremely thin, the energy density after absorption conversion can be very high and can be used for high-sensitivity detection. Therefore, it is necessary to propose a perfect absorber with a simple structure and light weight. Summary of the invention

[0004] Purpose of the invention: In view of the defects of the current perfect absorber, such as complex structure, heavy device, high loss and unfavorable integration, the present invention proposes a new perfect absorber and its preparation method. The perfect absorber of the present invention is an ultra-thin asymmetric perfect absorber, which not only has a simple structure, but also has a metal layer thickness of between 0.2 and 1 / 1000 of the entire perfect absorber.

[0005] In order to achieve the above invention purpose, the first technical solution proposed by the present invention is a perfect absorber, including a cylindrical silicon array layer and an ultra-thin lossy metal film layer on the array. The thickness of the lossy metal layer is only about one twenty-five thousandth of the target wavelength.

[0006] The second technical solution provided by the present invention is a method for preparing a perfect absorber, comprising the following steps:

[0007] 1. Growth of metal film: Clean the SOI substrate and grow a metal film on the surface of the SOI substrate using magnetron sputtering;

[0008] 2. Spin-coating photoresist: Spin-coating photoresist on the surface of the SOI substrate on which the metal film has been grown and baking;

[0009] 3. UV exposure and development: Place the SOI substrate with spin-coated photoresist on the photolithography machine and align it with the mask. The structure of the mask used is a periodic circular structure. After exposure, develop it with a positive photoresist developer and then post-bake it;

[0010] 4. Etching the metal film layer: Use reactive ion etching process to etch a periodic disk structure on the metal film, clean the remaining photoresist with acetone, clean it with alcohol and deionized water, and then dry it;

[0011] 5. Repeat steps 3 and 4, but the structure of the mask used is a silicon pillar structure;

[0012] 6. Etching SOI substrate: Use deep silicon etching process to etch silicon cylindrical structure on SOI substrate, clean the residual photoresist with acetone, clean with alcohol and deionized water and dry;

[0013] 7. Spin coating two layers of photoresist: Spin coating two layers of photoresist in sequence on the back side of the SOI substrate;

[0014] 8. Repeat steps 3 and 4, but the structure of the mask used is a square structure;

[0015] 9. Repeat step 6 to completely etch away the remaining portion of the SOI substrate;

[0016] 10. Release structure: Use buffered oxide etchant to etch away the remaining silicon dioxide layer on the SOI substrate, and finally obtain a perfect absorber.

[0017] Beneficial effects: The present invention discloses an ultra-thin asymmetric perfect absorber, which can achieve asymmetric perfect absorption in the terahertz band and the infrared band. Compared with the traditional perfect absorber, the thickness of the metal layer is much smaller than the working wavelength, so it has the advantages of small size, low loss, and high integration. In addition, due to the characteristics of low heat capacity and low thermal conductivity of the perfect absorber, it also has application prospects in the measurement of radiation heat. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a schematic diagram of the structure of the perfect absorber of the present invention;

[0019] Figure 2(a) and Figure 2(b) are Figure 1 Top and side views of the

[0020] FIG3(a) and FIG3(b) are respectively a simulation result diagram and an experimental result diagram of the S parameter curve of Example 1 in the terahertz band;

[0021] FIG. 4( a ) and FIG. 4( b ) are respectively a simulation result diagram and an experimental result diagram of the S parameter curve of Example 2 in the terahertz band;

[0022] FIG. 5( a ) and FIG. 5( b ) are respectively a simulation result diagram and an experimental result diagram of the S parameter curve of Example 3 in the terahertz band;

[0023] Figure 6 is a diagram of the absorption rate of Example 2 in the terahertz band at different incident directions;

[0024] FIG7( a ) and FIG7( b ) are respectively the reverse incidence diagram and the forward incidence diagram of the power loss density of the absorber of Example 2;

[0025] Figure 8 This is a scanning electron microscope image of Example 2. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is now further described in conjunction with the accompanying drawings and embodiments.

[0027] like Figure 1 As shown in FIG. 2(a) and FIG. 2(b), the perfect absorber of the present invention comprises: a silicon cylindrical unit 1, an ultra-thin lossy metal layer 2, and a silicon connecting bridge 3. The ultra-thin lossy metal layer 2 is located on the upper surface of the silicon cylindrical unit 1 and directly contacts the silicon cylindrical unit 1, and has a concentric structure. The silicon cylindrical units 1 are connected by the silicon connecting bridge 3 to form a unit structure.

[0028] The present invention designs the thickness of the lossy metal layer to be much smaller than the target wavelength, and does not set a backplate metal layer, thereby preparing an extremely thin terahertz unidirectional perfect absorber, greatly simplifying the absorber structure, improving process compatibility, and thus expanding the application field.

[0029] Examples 1-3

[0030] Step 1: Use acetone and ethanol low-power ultrasonic cleaning to clean the silicon wafer respectively; blow dry the cleaned silicon wafer with nitrogen to obtain a high-cleanliness silicon wafer; and grow a lossy metal layer with good uniformity on the silicon wafer by magnetron sputtering.

[0031] Step 2: Spin-coat positive photoresist on the surface of the metal layer; use a photolithography machine to perform patterned exposure on the positive electron beam resist; obtain a positive photoresist mask with a disc shape through development and fixing treatment; use reactive ion etching to remove excess lossy metal film to obtain a lossy metal disc unit.

[0032] Step 3: Use acetone and ethanol low-power ultrasound to clean the silicon wafer in turn; blow dry the cleaned silicon wafer with nitrogen to obtain a high-cleanliness sample.

[0033] Step 4: Spin-coat positive photoresist on the surface of the metal layer; use a photolithography machine to perform patterned exposure on the positive electron beam resist; obtain a positive photoresist mask of the silicon cylindrical array unit through development and fixing treatment; use deep silicon etching to remove excess silicon to obtain the silicon cylindrical array unit.

[0034] Step 5: Use acetone and ethanol low-power ultrasound to clean the silicon wafer in turn; blow dry the cleaned silicon wafer with nitrogen to obtain a high-cleanliness sample.

[0035] Step 6: Spin-coat positive photoresist on the back of the sample; use a photolithography machine to perform patterned exposure on the positive electron beam resist; obtain a square positive photoresist mask through development and fixing processes; use deep silicon etching to remove excess silicon on the back of the sample to obtain a complete, extremely thin, unidirectional, perfect absorber.

[0036] After the above steps, the Figure 1 The extremely thin one-way perfect absorber shown.

[0037] The lossy metal disk material in the present embodiment 1 is niobium nitride material (NbN) with a radius of 10 microns. The height of the silicon cylinder is 80 microns and the radius is 102 microns. The height of the silicon connecting bridge is 80 microns and the line width is 10 microns. The lossy metal disk material in the present embodiment 2 is niobium nitride material (NbN) with a radius of 28.5 microns. The height of the silicon cylinder is 80 microns and the radius is 102 microns. The height of the silicon connecting bridge is 80 microns and the line width is 10 microns. The lossy metal disk material in the present embodiment 3 is niobium nitride material (NbN) with a radius of 40 microns. The height of the silicon cylinder is 80 microns and the radius is 102 microns. The height of the silicon connecting bridge is 80 microns and the line width is 10 microns. The absorbers prepared in the present embodiments 1-3 were tested, and the experimental results are as follows. Figure 3(a)-6 As shown, it is basically consistent with the simulation results. At the same time, it can be seen that the radius of the ultra-thin lossy metal disk of the absorber has a very large influence on the absorption performance. When the radius of the ultra-thin lossy metal disk is 28.5 microns, asymmetric perfect absorption can be achieved. The power loss density of the perfect absorber prepared in Example 2 is simulated, and the simulation results are shown in Figure 7 (a) and Figure 7 (b). It can be seen that all losses generated by the perfect absorber during operation are concentrated in the ultra-thin lossy metal layer and will not affect other structures in the unit. The final physical picture is shown in Figure 8 shown.

[0038] The above are embodiments of the present invention and do not limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention are included in the protection scope of the present invention.

Claims

1. A perfect absorber, characterized in that: include: A silicon cylindrical unit, an ultra-thin lossy metal layer and a silicon connecting bridge, wherein the ultra-thin lossy metal layer is located on the upper surface of the silicon cylindrical unit and is in direct contact with the silicon cylindrical unit, belonging to a concentric structure, and the silicon connecting bridge connects the silicon cylindrical units to form a unit structure; the thickness of the lossy metal layer is between 0.2 and 1 / 1000 of the entire perfect absorber, and all losses generated when the perfect absorber is working are concentrated in the ultra-thin lossy metal layer, which will not affect other structures in the unit; and the perfect absorber has no backplane metal layer.

2. A method for preparing the perfect absorber as claimed in claim 1, characterized in that: The following steps are involved: (1) growing a metal film: cleaning the SOI substrate and growing a metal film on the surface of the SOI substrate by magnetron sputtering; (2) spin coating a photoresist: spin coating a photoresist on the surface of the SOI substrate on which the metal film has been grown and baking; (3) UV exposure and development: Place the SOI substrate with spin-coated photoresist on the photolithography machine and align it with the mask. The structure of the mask used is a periodic circular structure. After exposure, develop it with a positive photoresist developer and then post-bake it. (4) Etching the metal film layer: using a reactive ion etching process to etch a periodic disk structure on the metal film, cleaning the remaining photoresist with acetone, cleaning with alcohol and deionized water, and then drying; (5) Repeating the steps (3) and (4), but the structure of the mask used is a silicon column structure; (6) Etching the SOI substrate: using a deep silicon etching process to etch a silicon column structure on the SOI substrate, cleaning the remaining photoresist with acetone, cleaning with alcohol and deionized water, and then drying; (7) Spin coating two layers of photoresist: on the back of the SOI substrate, spin coating two layers of photoresist in sequence; (8) Repeating the steps (3) and (4), but the structure of the mask used is a square structure; (9) Repeating the step (6) to completely etch away the remaining part of the SOI substrate; (10) Releasing the structure: using a buffered oxide etchant to etch away the remaining silicon dioxide layer of the SOI substrate, and finally obtaining a perfect absorber.

3. The method according to claim 2, characterized in that The metal film layer is a lossy metal layer.

Images