Focusing type high-extinction-ratio infrared polarization integrated detector structure and design method thereof

Abstract

The invention discloses a focusing type high-extinction-ratio infrared polarization integrated detector structure and a design method thereof. The focusing type high-extinction-ratio infrared polarization integrated detector structure comprises a reading circuit layer, an indium column interconnection layer, a photosensitive chip absorption layer, an air spacing layer, an integrated indium column layer, a metal grating layer, a substrate layer and a super-structure lens condensation layer. Through the double-sided alignment design of the super-structure lens condensation layer and the metal grating layer, the infrared polarization detector has the functions of light convergence and polarization filtering corresponding to linearly polarized light, and realizes a high extinction ratio after being aligned and integrated with a focal plane pixel.

Description

Technical Field

[0001] The invention belongs to the field of infrared polarization imaging, and in particular relates to a focusing high extinction ratio infrared polarization integrated detector structure and a design method thereof. Background Art

[0002] Infrared detection technology has shown extremely important and extensive application value in the fields of space remote sensing, search and identification, reconnaissance and tracking, missile precision guidance and medical detection. However, with the rapid development of metamaterial stealth technology and camouflage technology, as well as the increasingly complex and changeable detection environment, the traditional infrared detection technology, which mainly focuses on improving the intensity of light response, has gradually failed to meet the current detection needs.

[0003] In order to meet this challenge, with the continuous progress of advanced nano-processing technologies such as electron beam lithography, holographic exposure, and nanoimprinting, pixel-level (i.e., sub-focal plane) polarization integrated infrared detectors have emerged. This type of detector can largely overcome a series of shortcomings inherent in system-level and component-level infrared polarization detectors, such as large size, heavy weight, high energy consumption, and complex image registration. Pixel-level polarization integrated infrared detectors can achieve simultaneous and co-local polarization imaging of the same dynamic target, without the need for image registration during image information fusion processing, thereby significantly improving detection efficiency and accuracy. In addition, its advantages of small size, light weight, and low energy consumption further broaden its scope of application. For example, patent document CN107342343 discloses an infrared polarization focal plane device structure and its preparation method, which can obtain polarization images of scene light waves in real time, avoiding the bulkiness and high cost of traditional equipment, and has high practicality and economy. However, due to the limitations of the pixel center distance size and the working wavelength, this type of polarization integrated detector is prone to light crosstalk between adjacent pixels in different polarization directions, resulting in a low focal plane extinction ratio, which seriously affects the imaging quality.

[0004] Therefore, how to effectively solve the optical crosstalk problem and improve the focal plane extinction ratio has become a key technical issue that needs to be urgently solved in this field. Summary of the invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the present invention proposes a focusing high extinction ratio infrared polarization integrated detector structure and a design method thereof, which vertically integrates the meta-lens, metal grating and detector in a pixel-level aligned stack, effectively weakens the signal diffraction crosstalk between adjacent pixels and improves the polarization extinction ratio, and can realize the simultaneous collection and imaging of information in different polarization directions of the target.

[0006] The technical solution of the present invention is as follows: On the one hand, the present invention provides a focused high extinction ratio infrared polarization integrated detector structure, which is characterized by comprising: bottom readout circuit layer; An indium pillar interconnection layer, disposed above the bottom readout circuit layer; A photosensitive chip absorption layer, interconnected with the bottom readout circuit layer through the indium pillar interconnection layer, jointly forming a photosensitive infrared detection chip, including photosensitive elements for absorbing infrared light; An air spacer layer, disposed above the photosensitive chip absorption layer; An integrated indium pillar layer, disposed above the air spacer layer, for providing structural support and electrical connection; A metal grating layer, disposed above the integrated indium pillar layer, including linear gratings with four different polarization orientations, for performing polarization filtering on infrared light; A substrate layer, disposed above the metal grating layer, providing support for the metal grating layer; A metasurface lens light condensing layer, disposed on the other side of the substrate layer, aligned with the metal grating layer, composed of a micro-structure array of different sizes, each unit lens corresponding one-to-one with the linear gratings of different polarization orientations and the photosensitive elements in the photosensitive chip absorption layer, and located directly above the corresponding photosensitive element to completely cover it; Wherein, the thickness of the air spacer layer is matched with the focal length of the metasurface lens light condensing layer. Through the double-sided pixel alignment design of the metasurface lens light condensing layer and the metal grating layer, it has the functions of light convergence for corresponding linearly polarized light and filtering of different polarization directions, so as to form an infrared polarization detector with a high extinction ratio after being integrated with the focal plane pixels.

[0007] Preferably, the dielectric column unit cell structure of the metasurface lens light condensing layer uses the finite-difference time-domain method (FDTD) or the finite element method (FEM) to calculate the corresponding phase magnitudes after the incident light acts on each dielectric column unit cell, and obtain the relationships between the phase, transmittance and different dielectric column diameters or duty cycles.

[0008] Preferably, the dielectric column unit cell parameters of the micro-structures of the metasurface lens light condensing layer mainly include the array period, the duty cycle of the dielectric column, the height of the dielectric column, etc., and their arrangement regulates the phase according to the Fresnel hyperbolic law to achieve the focusing of light and the regulation of polarized light. The formula is: , Where x 、 y are the positions of the metasurface lens unit cells, λ is the target wavelength, n is the refractive index of the material background, f is the designed focal length, is the initial phase constant.

[0009] Preferably, the unit structure of the metasurface lens light condensing layer is a centrosymmetric structure and is prepared from an optical dielectric material without absorption in the corresponding band.

[0010] Preferably, the photosensitive chip absorption layer and the metal grating layer are interconnected through an integrated indium pillar layer.

[0011] Preferably, the metal grating layer is disposed on the integrated indium pillar layer and includes linear gratings with four different polarization orientations, the angles of which are 0°, 45°, 90°, and 135° in sequence. The material of the metal grating is Au or Al, the grating period is less than 1 μm, the duty cycle of the metal line width ranges from 0.4 to 0.6, and the grating height ranges from 0.3 to 0.5 μm.

[0012] On the other hand, the present invention also provides a design method for an infrared high extinction ratio polarization detector structure, which is characterized by including: Designing a readout circuit layer; Designing and fabricating an indium pillar interconnection layer on the readout circuit layer; Designing and fabricating a photosensitive chip absorption layer on the indium pillar interconnection layer; Designing and fabricating an air spacer layer on the photosensitive chip absorption layer, the thickness of which is matched with the focal length of the subsequent metasurface condenser layer; Designing and fabricating a metal grating layer including a metal grating array structure with four different polarization orientations above the air spacer layer or above the integrated indium pillar layer; Selecting silicon (Si) or germanium (Ge) as the substrate material, and designing and fabricating a metasurface condenser layer on the other side of the substrate. The metasurface condenser layer is composed of an array of microstructures with different sizes obtained by etching; Ensuring that the metal grating layer and the metasurface condenser layer with each angular orientation correspond one-to-one with the photosensitive elements in the photosensitive chip absorption layer and are located directly above them; Integrating the metal grating layer and the metasurface condenser layer with the photosensitive chip absorption layer through a double-sided alignment technique to form an infrared high extinction ratio polarization detector with light convergence and polarization filtering functions.

[0013] Furthermore, it also includes optimization design steps for the materials, sizes, and structures of the readout circuit layer, indium pillar interconnection layer, photosensitive chip absorption layer, air spacer layer, metal grating layer, and metasurface condenser layer to ensure the performance of the detector.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. By adopting a metasurface, effective filtering and convergence of different linearly polarized lights are achieved within the pixel space, significantly reducing the optical crosstalk problem between different polarization angles in the prior art, improving the polarization extinction ratio of the device, and ensuring the accuracy and reliability of the detection results.

[0015] 2. Fabricating sub - micron scale structures such as metasurface lenses and metal gratings on both sides of an independent substrate has the advantage of simple process technology. Meanwhile, damage to the infrared focal plane detector is avoided during the processing, thereby improving the yield of the polarization integrated detector. In addition, this processing method also reduces production costs and improves production efficiency, providing strong support for large - scale production.

[0016] 3. The detector adopts a stacked design, integrating a metasurface lens, a metal grating and a photosensitive chip into one body, achieving a high extinction ratio polarization detection ability. There is no need to use independent optical elements, and it has the advantages of high integration, easy preparation, small volume, high stability and reliability. Brief Description of the Drawings

[0017] Figure 1 is a cross - sectional view of the structure of the focusing high - extinction - ratio infrared polarization integrated detector of the present invention; Figure 2 is a top - view of the metal grating layer in the present invention; Figure 3 is a schematic diagram of an embodiment of the metasurface lens light - condensing layer of the present invention. Among them, (a) the cell unit in the metasurface lens structure; (b) the relationship between phase, transmittance and the radius of the nanorod; (c) the top - view of the 2×2 lens unit in the metasurface lens structure; (d) the light field convergence effect after passing through the metasurface lens; Figure 4 is the relationship between the polarization extinction ratio and the integration distance between the grating array and the photosensitive element of the focal plane. Detailed Embodiments

[0018] To better understand the present invention, the content of the present invention will be further clarified below in conjunction with embodiments and drawings. However, the present invention is not limited to the following embodiments.

[0019] Please refer to Figure 1 , Figure 1 is a schematic diagram of an embodiment of the structure of the focusing high - extinction - ratio infrared polarization integrated detector of the present invention. As shown in the figure, the structure of the focusing high - extinction - ratio infrared polarization integrated detector is composed of multiple functional layers from bottom to top, and each layer works together to achieve the functions of polarization detection and light convergence of infrared light. The specific structure is as follows; The bottom read - out circuit layer 1, as the bottom layer of the detector, is used to read and process the electrical signals generated by the photosensitive chip absorption layer 3; The indium pillar interconnection layer 2 is used to connect the bottom read - out circuit layer 1 and the photosensitive chip absorption layer 3 to achieve the transmission of electrical signals.

[0020] The photosensitive chip absorption layer 3 is used to absorb infrared light and generate corresponding electrical signals, and is prepared by a conventional wafer - processing technology.

[0021] The air spacer layer 4 provides a certain spatial interval for optimizing the optical path and reducing interlayer interference.

[0022] The integrated indium pillar layer 5 is used for the integration between the photosensitive chip absorption layer 3 and the metal grating layer 6 to ensure electrical connection and optical coupling.

[0023] The metal grating layer 6 is used for polarization filtering of mid-wave infrared light, and realizes the selective transmission of light with different polarization directions by designing specific structural parameters. As Figure 2 shown, in this embodiment, the grating material of the metal grating layer 6 is Au or Al, the grating period p 0 is less than 1 μm, the grating line width w 0 duty cycle ranges from 0.4 to 0.6, and the grating height h 0 ranges from 0.3 to 0.5 μm. A linear grating with four different polarization orientations (0°, 45°, 90°, and 135°) is used to achieve polarization filtering in different directions.

[0024] The metasurface lens condenser layer 8 converges mid-wave infrared light to improve the light collection efficiency. It can be composed of microstructures of different sizes or replaced by a hemispherical microlens array. As Figure 3 shown, in the unit cell structure aligned with the metal grating layer 6, the period of the dielectric column unit cell p 1 ranges from 1 to 3 μm, the radius of the dielectric column r 1 (duty cycle) ranges from 0.1 to 0.8, and the height of the dielectric column h 1 ranges from 1 to 10 μm. By adjusting these parameters, the optical performance of the metasurface lens can be optimized.

[0025] The substrate layer 7, made of germanium or silicon, is used to support the metal grating layer 6 and the metasurface lens condenser layer 8 and ensure their alignment.

[0026] The metal grating layer 6 and the metasurface lens condenser layer 8 are respectively designed and fabricated on both sides of the substrate layer 7 made of germanium or silicon. Each unit lens is aligned with the polarization grating with different polarization orientations. The thickness range of the substrate layer 7 is within 100 - 400 μm.

[0027] The unit cell structure of the metasurface lens condenser layer 8 aligned with the metal grating layer 6 is as Figure 3 shown in (a) of the figure. The period of the dielectric column unit cell ranges from 1 to 3 μm, the duty cycle of the dielectric column ranges from 0.1 to 0.8, and the height of the dielectric column ranges from 1 to 10 μm. Under the condition of selecting the period and height of the dielectric column unit cell, the finite-difference time-domain (FDTD) method or the finite element method (FEM) is used to calculate the corresponding phase size after the incident light interacts with each dielectric column unit cell, and the relationship between the phase, transmittance and different dielectric column diameters or duty cycles is obtained.

[0028] The metasurface lens condenser layer 8 is composed of microstructures of different sizes, such as Figure 3 shown in (c), and the arrangement regulates the phase according to the Fresnel hyperbolic law to achieve the light focusing effect. The formula is: , where x, y is the position of the metasurface lens cell, λ is the target wavelength, n is the refractive index of the material background, f is the designed focal length, and C( ) is the initial phase constant.

[0029] The metasurface lens condenser layer 8 array can also be replaced by a hemispherical microlens array to achieve the light focusing effect.

[0030] The material of the substrate layer 7 is germanium or silicon, and the thickness range is within 100 - 400 μm, which is used to support the metal grating layer 6 and the metasurface lens condenser layer 8 and ensure the alignment between them.

[0031] The integration of the photosensitive chip absorption layer 3 and the metal grating layer 6 is carried out using an integrated indium pillar layer 5 to ensure the optical coupling between the photosensitive chip absorption layer 3 and the metal grating layer 6.

[0032] Example: For the polarization filtering and light focusing laminated structure parameter determination and overall structure design of mid - wave infrared light in the 3 - 5 μm band, the specific steps are as follows: Structural parameter design of the photosensitive chip absorption layer 3: The photosensitive element size is 30 μm × 30 μm, which is used to absorb and convert mid - wave infrared light signals.

[0033] Metal grating layer 6: Located on one side of the Si substrate, the coverage area for each polarization direction is 30 μm × 30 μm, aligned with the metasurface lens condenser layer 8, the material is selected as Al, and the polarization directions are 0°, 45°, 90°, 135°, as Figure 2 shown. The structural parameters for a single polarization angle are all grating period p 0 = 400 nm, grating line width w 0 = 200 nm (duty cycle is 0.5), and grating height h 0 = 300 nm, ensuring that the extinction ratio for a single polarization angle is greater than 500:1.

[0034] Metasurface lens condenser layer 8: Located on the other side of the Si substrate, aligned with the metal grating layer. The thickness of the Si substrate is 200 μm. The nano - unit structure is as Figure 3 shown in (a), and the design method is as follows: Determine the period of the nano - unit p 1. The radius of the nano - pillar r1 (Duty cycle), column height h 1 and phase phase and transmittance T of the dispersion relation, as Figure 3 shown in (b) below. The nano-unit parameters of this embodiment p 1 = 1μm, 150nm ≤ r 1≤400nm, h 1 = 1μm.

[0035] By the formula to determine the phase distribution requirements at different spatial positions (x,y), determine the combination of nano-columns with different radii, and obtain the basic parameters of the metasurface lens, as Figure 3 shown in (c) below.

[0036] Converge the light with a central wavelength of 4 μm, and the effect is as Figure 3 shown in (d) below. The focal length is 217μm, close to the designed value of 220μm.

[0037] The stacked structure integration uses a 200μm-thick Si substrate to design a double-sided alignment metasurface lens and a metal grating array. After the integration of the metasurface lens - metal grating array structure and the focal plane pixel alignment, as Figure 1 shown below. After integration, the relationship between the polarization extinction ratio and the integration distance of the grating surface from the photosensitive element is as Figure 4 shown below. The results show that when the integration distance is 20μm, the polarization extinction ratio of the focal plane integrated metasurface lens light condensing structure is 35:1, which is about 1 time higher than that without the integrated metasurface lens light condensing structure.

[0038] In this embodiment, through the collaborative design of the metasurface lens and the metal grating, the optical crosstalk between different polarization angles is significantly reduced, and the polarization extinction ratio is increased to 35:1 (when the integration distance is 20μm), which is about 1 time higher than that of the traditional structure. The light condensing layer of the metasurface lens realizes the precise focusing of light through the microstructure array, improves the light collection efficiency, enhances the detection sensitivity, and realizes the polarization filtering and light convergence of mid-wave infrared light. The metasurface lens and the metal grating are independently processed on both sides of the substrate, avoiding damage to the infrared focal plane detector, improving the yield rate, reducing the production cost at the same time, and being suitable for large-scale production. By optimizing the structural parameters of the metasurface lens and the metal grating through the finite-difference time-domain method (FDTD) or the finite element method (FEM), the performance can be adjusted according to different band requirements, and the adaptability is wide. In summary, the present invention realizes high extinction ratio and efficient light convergence infrared polarization detection through multi-layer integration and double-sided alignment, solves the problems of serious optical crosstalk and large volume of traditional detectors, and provides a new technical approach for infrared polarization detection.

Claims

1. A focusing type infrared polarization integrated detector structure with a high extinction ratio, characterized in that, Including, successively arranged from bottom to top: A bottom readout circuit layer (1) for signal reading and processing; An indium pillar interconnection layer (2) bonded to the bottom readout circuit layer (1) through a flip-chip bonding process; A photosensitive chip absorption layer (3) electrically interconnected with the bottom readout circuit layer (1) through the indium pillar interconnection layer (2), including a plurality of photosensitive elements arranged in an array for absorbing infrared light; An air spacer layer (4) disposed on the surface of the photosensitive chip absorption layer (3); An integrated indium pillar layer (5) disposed on the surface of the photosensitive chip absorption layer (3) for providing structural support and fixed connection; A metal grating layer (6) disposed on the integrated indium pillar layer (5), including a plurality of line gratings arranged in an array, each line grating having four different polarization orientations of 0°, 45°, 90°, and 135°, and each line grating aligned with the photosensitive element below for polarization filtering of infrared light; A substrate layer (7) disposed on the metal grating layer (6) to provide support for the metal grating layer (6); A metasurface lens focusing layer (8) disposed on the other side of the substrate layer (7), including a plurality of lens units arranged in an array, and the phase distribution of the lens units satisfying the Fresnel hyperbola regulation law to achieve phase regulation and focusing of light; Wherein, each lens unit of the metasurface lens focusing layer (8) forms a vertical optical path channel with the line grating in the corresponding direction of the metal grating layer (6) and the photosensitive element of the photosensitive chip absorption layer (3); the thickness of the air spacer layer (4) is matched with the focal length of the metasurface lens focusing layer (8), and by controlling the thickness of the air spacer layer (4), the incident light is focused by the metasurface lens focusing layer (8) and converges within the effective area of the corresponding photosensitive element, thereby suppressing the optical crosstalk between adjacent pixels and achieving a high polarization extinction ratio.

2. The structure of the focusing high extinction ratio infrared polarization integrated detector according to claim 1, wherein The lens unit of the metasurface lens condensing layer (8) comprises n cell structures, with a dielectric column as the core. The arrangement period of the dielectric columns in the horizontal direction is p 1, and the periods in two orthogonal directions are the same.

3. The structure of the focused high extinction ratio infrared polarization integrated detector according to claim 2, characterized in that The period size range of the unit structure dielectric columns of the metasurface lens focusing layer (8) is 1 - 3 μm, the duty cycle range of the dielectric columns is 0.1 - 0.8, and the height range of the dielectric columns is 1 - 10 μm. It is designed by the finite difference time domain method or the finite element method, and its corresponding phase distribution follows the Fresnel hyperbola law: , wherein x, y is the position of the meta-lens unit cell, λ is the target wavelength, n is the refractive index of the material background, f is the designed focal length, C( ) is the initial phase constant.

4. The structure of the focusing high extinction ratio infrared polarization integrated detector according to claim 1, characterized in that, The unit structure of the metasurface lens focusing layer (8) is a centrosymmetric structure and is prepared from an optical dielectric material without absorption in the corresponding wavelength band.

5. The structure of the focused high extinction ratio infrared polarization integrated detector according to claim 1, characterized in that The metal grating material of the metal grating layer (6) is Au or Al, the grating period is less than 1 μm, the duty cycle range of the metal grating line width is 0.4 - 0.6, and the grating height range is 0.1 - 0.5 μm.

6. A structural design method for a focused high extinction ratio infrared polarization integrated detector, characterized in that Including: Designing the bottom readout circuit layer (1); Designing and fabricating the indium pillar interconnection layer (2) on the bottom readout circuit layer (1); Designing and fabricating the photosensitive chip absorption layer (3) on the indium pillar interconnection layer (2); Designing and fabricating the air spacer layer (4) on the photosensitive chip absorption layer (3), the thickness of which is matched with the focal length of the subsequent metasurface lens focusing layer (8); Selecting silicon or germanium as the substrate material and designing and fabricating the metal grating layer (6) with a metal grating array structure including four different polarization orientations on one side of the substrate; On the other side of the substrate, a metasurface lens condenser layer (8) is designed and fabricated, and the metasurface lens condenser layer (8) is composed of an array of microstructures with different sizes obtained by etching; Through double-sided alignment technology, it is ensured that the metal grating layers (6) and the metasurface lens condenser layer (8) units corresponding to each angular orientation are in one-to-one correspondence; Above the air spacer layer (4), the metal grating layer (6) and the metasurface lens condenser layer (8) are integrated with the photosensitive chip absorption layer (3) through an integrated indium pillar layer (5) to form a high extinction ratio infrared polarization detector with both light convergence and polarization filtering functions.

7. The structural design method of a focused high extinction ratio infrared polarization integrated detector according to claim 6, characterized in that, It also includes steps of optimizing the design of the materials, sizes and structures of the bottom readout circuit layer (1), indium pillar interconnect layer (2), photosensitive chip absorption layer (3), air spacer layer (4), metal grating layer (6), and metasurface lens condenser layer (8) to ensure the performance of the detector.

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