CN112014915A

CN112014915A - Multilayer symmetrical two-dimensional transmission grating with adjustable 10-14 micron central wavelength and preparation method thereof

Info

Publication number: CN112014915A
Application number: CN201910462848.9A
Authority: CN
Inventors: 赵永强; 刘芯羽; 汤超龙
Original assignee: Shenzhen Institute of Northwestern Polytechnical University
Current assignee: Shenzhen Institute of Northwestern Polytechnical University
Priority date: 2019-05-30
Filing date: 2019-05-30
Publication date: 2020-12-01
Anticipated expiration: 2039-05-30
Also published as: CN112014915B

Abstract

The invention discloses a multilayer symmetrical two-dimensional transmission grating with adjustable 10-14 micron central wavelength and a preparation method thereof, which solve the problem of processing errors of an FP (Fabry-Perot) cavity optical filter in a long-wave infrared band due to complex film systems. The invention comprises the following steps: the grating layer, the substrate layer and the symmetrical high-reflection layer; the symmetrical high-reflection layers are 8 layers from top to bottom, and the base layer is the bottommost layer of the symmetrical high-reflection layers; the symmetrical high-reflection layers are alternately arranged by metal materials and dielectric materials; the metal material is germanium with a high refractive index, and the medium material is yttrium fluoride with a low refractive index; the grating layer is made of high-refractive-index material germanium, in the symmetrical high-reflection layer, the refractive-index material germanium and the low-refractive-index material yttrium fluoride are alternately arranged, and the thickness of the high-reflection layer is one quarter wavelength; the base layer is made of germanium; the grating layer is positioned between the fourth layer and the fifth layer which are arranged from top to bottom in the high reflection layer.

Description

Multilayer symmetrical two-dimensional transmission grating with adjustable 10-14 micron central wavelength and preparation method thereof

Technical Field

The invention belongs to the technical field of design of micro-nano devices, and relates to a multilayer symmetrical two-dimensional transmission grating structure with adjustable central wavelength of 10-14 micrometers.

Background

The hyperspectral imaging technology based on the optical filter can quickly acquire high-resolution images in real time, so that the hyperspectral imaging technology is widely applied to real-time detection and classification. The main technique of spectral imaging at present is pixel chip coating, and is mostly applied to visible light, near infrared and mid-infrared bands. However, the long-wave infrared band mostly passes through the center wavelength of a specific band by changing the cavity depth of the FP cavity or filling media. Generally, the film system is complex and has more layers, and the increase of the thickness of the film layer in the preparation process can cause stress deformation, which brings difficulty to the preparation process and finally causes the increase of errors. Therefore, the optical filter with a simple structure designed in the long-wave infrared band has important theoretical significance and practical significance.

In recent years, a sub-wavelength grating has been widely used as a filter film in spectral spectroscopy, and a guided-mode resonance sub-wavelength filter device has been receiving much attention in recent years because of its advantages such as extremely narrow bandwidth, extremely high diffraction efficiency, and low sideband effect. Under the condition that the grating structure parameters and the incidence conditions are specific, the guided mode resonance effect enables the spectrum to have the characteristics of narrow bandwidth, low side band and high diffraction rate, and the central wavelength can be tuned by adjusting the grating period, the duty ratio and the thickness, so that the degree of freedom of central wavelength adjustability is increased.

Disclosure of Invention

The invention aims to provide a multilayer symmetrical two-dimensional transmission grating with the duty ratio of the grating being between 0 and 1 and between 10 and 14 microns, and solves the problem of processing errors of an FP (Fabry-Perot) cavity optical filter in a long-wave infrared band due to complex film systems.

In order to achieve the purpose, the invention adopts the following technical scheme to realize the purpose:

a10-14 micron multilayer symmetric two-dimensional transmission grating with adjustable center wavelength comprises: the grating layer, the substrate layer and the symmetrical high-reflection layer; the symmetrical high-reflection layers are 8 layers from top to bottom, and the base layer is the bottommost layer of the high-reflection layers; .

The symmetrical high-reflection layers are alternately arranged by metal materials and dielectric materials; the metal material is germanium with a high refractive index, and the medium material is yttrium fluoride with a low refractive index; the grating layer is made of high-refractive-index material germanium, in the symmetrical high-reflection layer, the refractive-index material germanium and the low-refractive-index material yttrium fluoride are alternately arranged, and the thickness of the symmetrical high-reflection layer is one quarter wavelength;

the base layer is made of germanium;

the grating layer is positioned between the fourth layer and the fifth layer which are arranged from top to bottom in the symmetrical high reflection layer.

As a preferred embodiment of the present invention: the thickness of the grating layer is 0.58-2.4 μm.

As a preferred embodiment of the present invention: the grating layer comprises two-dimensional square blocks which are uniformly distributed, the duty ratios of the grating layer are respectively 0.1, 0.6, 0.8 and 0.92, and the sizes of the corresponding two-dimensional square blocks are respectively 300nm multiplied by 300nm, 1.8 mu m multiplied by 1.8 mu m, 2.4 mu m multiplied by 2.4 mu m and 2.76 mu m multiplied by 2.76 mu m.

As a preferred embodiment of the present invention: the high refractive index is a refractive index of greater than or equal to 4; the low refractive index is a refractive index of 1.5 or less.

The invention also discloses a preparation method of any of the gratings, which comprises the following specific preparation processes:

(1) film coating: preparing an optical film by ion beam sputtering deposition technology in a film coating mode on each layer; the ion beam sputtering deposition technology is a process of bombarding the surface of a solid by particles with certain energy to ensure that atoms or molecules near the surface of the solid obtain enough energy and finally escape from the surface of the solid; when a film is deposited, a sputtering source is arranged on a target and is sputtered after being bombarded by argon ions; if the target material is simple substance, generating a simple substance film of the target material on the substrate; if reaction gas is intentionally introduced into the sputtering chamber and reacts with sputtered target atoms to deposit on the substrate, a compound film of the target material can be formed; the particles are ions or neutral atoms, molecules;

(2) electron beam exposure: electron beam exposure is a microfabrication technique in which an electron beam of low power density is used to irradiate an electro-resist, and a pattern is generated in the resist after development; for the positive resist, the resist in the area irradiated by the electron beam is dissolved away after development, while the resist in the area not irradiated is remained; the opposite is true for negative resists; a uniform photoresist coating is one of the indispensable conditions for obtaining a high-quality sub-wavelength grating; coating a photoresist; using electron beam exposure to convert the pattern on the mask plate into a substrate surface dielectric pattern;

(3) the inductive coupling ion etching technology comprises the following steps: the inductively coupled ion etching technology is a semiconductor dry etching technology, reaction gas is introduced, inductively coupled plasma glow discharge is used for decomposing the reaction gas, the generated plasma with strong chemical activity moves to the surface of a sample under the acceleration action of an electric field, and the surface of the sample is subjected to chemical reaction to generate volatile gas and has a certain physical etching effect; and forming a notch groove with a periodic structure on the substrate by using an inductively coupled ion etching technology.

The invention has the beneficial effects that:

according to the invention, through the combined use of the metal material germanium commonly used for long-wave infrared and the dielectric material yttrium fluoride, a brand-new multilayer symmetrical two-dimensional grating structure is adopted, and the good band-pass characteristic and the low side band effect in the long-wave infrared band are realized. Meanwhile, the effect of tuning the center wavelength can be realized by changing the duty ratio.

Drawings

FIG. 1-1 is a schematic structural view of an embodiment of the present invention;

FIG. 1-2 is a schematic structural view of a symmetrical high reflective layer;

FIGS. 1-3 are schematic structural diagrams of grating layers;

FIG. 2 shows the variation of the center wavelength and bandwidth with duty cycle in the long-wavelength infrared 10-14 μm band.

FIG. 3 is a graph of the variation of center wavelength and bandwidth with period in the long-wave infrared 10-14 microns.

FIG. 4 is a graph of the center wavelength and bandwidth of the long-wave infrared 10-14 μm band as a function of the thickness of the high refractive index layer.

FIG. 5 shows the variation of the center wavelength and bandwidth with the thickness of the high refractive index layer and the low refractive index layer in the long-wave infrared 10-14 μm band.

FIGS. 6-1 to 6-4 are graphs of electric field distribution of the present invention when the light source wavelength is 10.26 micrometers, 11.2 micrometers, 12 micrometers, and 13 micrometers, respectively.

Description of reference numerals:

1-germanium, 2-yttrium fluoride, 3-high reflection layer, 4-grating layer and 5-substrate layer.

Detailed Description

The following description of the embodiments of the present invention refers to the accompanying drawings and examples:

as shown, the 10-14 micron center wavelength tunable multilayer symmetric two-dimensional transmission grating includes: a grating layer 4, a substrate layer 5 and a symmetrical high-reflection layer 3; the symmetrical high-reflection layers are 8 layers from top to bottom, and the base layer is the bottommost layer of the high-reflection layers;

the symmetrical high-reflection layers are alternately arranged by metal materials and dielectric materials; the metal material is germanium with a high refractive index, and the medium material is yttrium fluoride with a low refractive index; the grating layer is made of high-refractive-index material germanium, in the symmetrical high-reflection layer, the refractive-index material germanium and the low-refractive-index material yttrium fluoride are alternately arranged, and the thickness of the high-reflection layer is one quarter wavelength;

the base layer is made of germanium;

The high refractive index is a refractive index of greater than or equal to 4; the low refractive index is a refractive index of 1.5 or less.

As shown in the figure, the invention aims to provide a multilayer symmetrical two-dimensional transmission grating with the duty ratio of 0-1 freely adjustable between 10-14 microns, and solves the problem of processing errors of an FP (Fabry-Perot) cavity optical filter in a long-wave infrared band due to complex film systems.

The invention relates to a multilayer symmetrical two-dimensional transmission grating structure composed of metal and a dielectric material. In an embodiment of the present invention, the grating structure includes: grating layer, basal layer and symmetrical high-reflection layer (8 layers).

The invention provides a multilayer symmetrical two-dimensional transmission grating structure with tunable 10-14 micron central wavelength, which is mainly characterized in that a metal material and a dielectric material are combined for use, and a brand new two-dimensional grating structure is used, so that the central wavelength of a long-wave infrared 10-14 micron waveband can be adjusted by the thickness of a high reflection layer and the duty ratio of the two-dimensional structure.

Considering the problem that the transmittance of the one-dimensional sub-wavelength grating is low due to the polarization effect, designers adopt a two-dimensional grating structure. The grating layer is made of high-refractive-index material germanium, the symmetrical high-reflection layer is made of high-refractive-index material germanium and low-refractive-index material yttrium fluoride which are alternated, and the thickness of the symmetrical high-reflection layer is one quarter of the wavelength.

Material selection: the material of the grating layer is germanium; the basal layer is made of metal material germanium; the symmetrical high-reflectivity layer is formed by alternately arranging yttrium fluoride/germanium fluoride/yttrium fluoride/germanium. The materials have good transmission performance in long-wave infrared bands and can be prepared by the existing technical means.

The preparation method is combined with a micro-nano processing technology for preparation, and the specific preparation process comprises the following steps:

(1) film coating: the layers are coated in a film-coating mode, and an ion beam sputtering deposition technology is an important method for preparing a high-quality optical film. The method is a process for bombarding the surface of a solid with particles (ions or neutral atoms, molecules) with certain energy so that the atoms or molecules near the surface of the solid obtain enough energy to finally escape from the surface of the solid. Sputtering can only be performed under a certain vacuum condition. When the film is deposited, a sputtering source is arranged on a target and is sputtered after being bombarded by argon ions. If the target material is simple substance, generating a simple substance film of the target material on the substrate; if a reactive gas is intentionally introduced into the sputtering chamber to chemically react with sputtered target atoms and deposit them on the substrate, a compound thin film of the target material can be formed.

(2) Electron beam Exposure (EBL): electron beam exposure is a microfabrication technique in which an electron beam of low power density is irradiated to an electro-resist, which is developed to produce a pattern in the resist. For the positive resist, the resist in the area irradiated by the electron beam is dissolved away after development, while the resist in the area not irradiated is remained; the opposite is true for negative resists. Uniform photoresist coating is one of the indispensable conditions for obtaining high quality sub-wavelength gratings. After the photoresist is coated, the pattern on the mask is converted into a substrate surface dielectric pattern by electron beam exposure. The exposure mode has high resolution, easy mask plate manufacture, large process tolerance and high production efficiency, but because of the scattering of electron beams in the photoresist film, the exposure dose of the pattern can be influenced by the exposure dose of adjacent patterns (namely, proximity effect), and as a result, the line width is changed or the pattern is distorted after development.

(3) Inductively coupled ion etching (ICP): ICP is a very important semiconductor dry etching technology, reaction gas is introduced, inductively coupled plasma glow discharge is used for decomposing the reaction gas, the generated plasma with strong chemical activity moves to the surface of a sample under the acceleration action of an electric field, and the surface of the sample is subjected to chemical reaction to generate volatile gas and has a certain physical etching effect. Because the plasma source is separate from the rf acceleration source, the plasma density can be higher and the acceleration capability can be enhanced to achieve higher etch rates and better anisotropic etching. And forming a notch groove with a periodic structure on the substrate by using an inductively coupled ion etching technology.

In the following discussion, the following terms are defined:

duty ratio: the ratio of the width of the high index material germanium to the grating period.

Peak half width: the spectral response is the width of the waveform at half the transmission peak efficiency.

Peak transmittance: spectral transmittance corresponding to the center wavelength of the spectral response.

Side belts: if the spectral response is a single peak, taking the maximum transmission efficiency outside a half-width area of three times of peak values on the left side and the right side of the peak value; if the spectral response is multimodal, the sub-peak transmission efficiency is taken.

FIGS. 1-3 are schematic diagrams of a multilayer symmetrical two-dimensional grating structure according to the present invention. The middle layer is a grating layer, high-refractive-index material germanium is adopted, and grating structure parameters are designed through a guided mode resonance mechanism, so that the grating has the characteristics of narrow bandwidth and high reflectivity. The eight layers shown are symmetrical high reflectivity layers, in which the high index of refraction material is germanium and the low index of refraction material is yttrium fluoride, each of which is a quarter wavelength thick. The reflection grating is optimized into the transmission grating through the symmetrical high-reflection layer, so that the structure can finally realize extremely high transmission efficiency, extremely low side-band effect and excellent band-pass characteristic in a 10-14 micron wave band.

FIG. 2 is a graph of the effect on center wavelength and bandwidth of grating duty cycle varying from 0-1 in the long-wave infrared 10 μm-14 μm band. It can be seen from the figure that when the grating duty cycle is varied between 0-1, the center wavelength is significantly red-shifted with increasing duty cycle, the bandwidth increases and the transmittance increases. But all have extremely high peak transmissivity, extremely low side band effect and good band-pass characteristic.

In fig. 3, the change of the spectral response of the grating when the grating period is changed is given. It can be seen from the figure that when the grating period changes, the main evaluation parameters such as the center wavelength, the peak half-width, the side band and the like of the spectral response do not change significantly. In the actual processing and preparation process, an excessively narrow groove system is difficult to obtain, so that the grating structure provided by the invention adopts a design with a large period, and the preparation difficulty is reduced. Meanwhile, the period error in the processing process can not obviously change the performance of the grating.

In fig. 4, the change of the grating spectral response is given when the thickness of the symmetric high refractive index layer is changed. It can be seen from the figure that the grating has good bandpass characteristics between 10 μm and 14 μm when the grating layer thickness varies between 0.58 μm and 1.4 μm. As the thickness increases, the transmittance and peak half-width increase with the appearance of other peaks.

In fig. 5, the change of the grating spectral response when the thickness of the symmetric high reflective layer and the low refractive index layer is changed is shown. It can be seen that the center wavelength of the spectral response is substantially linear with thickness as the thickness varies from 1.58 μm to 2.4 μm. As the thickness increases, the peak half-width increases with a significant red-shift. This property can be used for the design and location of the center wavelength in the filter design process.

FIGS. 6-1 to-6-4 show the electric field distribution patterns of the structure at the light source wavelengths of 10.26 microns, 11.2 microns, 12 microns and 13 microns, respectively. Wherein the duty ratios of the grating layers are 0.1, 0.6, 0.8, 0.92, respectively, i.e. the sizes of the two-dimensional squares are 300nm × 300nm, 1.8 μm × 1.8 μm, 2.4 μm × 2.4 μm, 2.76 μm × 2.76 μm, respectively; it can be seen from the figure that when the light source is a designated light source, the two-dimensional structure can well transmit light of a desired wavelength band.

While the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, the present invention is not limited to the above embodiments, and various changes, which relate to the related art known to those skilled in the art and fall within the scope of the present invention, can be made within the knowledge of those skilled in the art without departing from the gist of the present invention.

Many other changes and modifications can be made without departing from the spirit and scope of the invention. It is to be understood that the invention is not to be limited to the specific embodiments, but only by the scope of the appended claims.

Claims

1.10-14 microns of multilayer symmetrical two-dimensional transmission grating with adjustable central wavelength, which is characterized in that: the method comprises the following steps: the grating layer, the substrate layer and the symmetrical high-reflection layer; the symmetrical high-reflection layers are 8 layers from top to bottom, and the base layer is the bottommost layer of the high-reflection layers;

the base layer is made of germanium;

2. The 10-14 micron central wavelength tunable multilayer symmetric two-dimensional transmission grating of claim 1, wherein: the thickness of the grating layer is 0.58-2.4 μm.

3. The 10-14 micron central wavelength tunable multilayer symmetric two-dimensional transmission grating of claim 1, wherein: the grating layer comprises two-dimensional square blocks which are uniformly distributed, the duty ratios of the grating layer are respectively 0.1, 0.6, 0.8 and 0.92, and the sizes of the corresponding two-dimensional square blocks are respectively 300nm multiplied by 300nm, 1.8 mu m multiplied by 1.8 mu m, 2.4 mu m multiplied by 2.4 mu m and 2.76 mu m multiplied by 2.76 mu m.

4. The 10-14 micron central wavelength tunable multilayer symmetric two-dimensional transmission grating of claim 1, wherein: the high refractive index is a refractive index of greater than or equal to 4; the low refractive index is a refractive index of 1.5 or less.

5. The method of making a grating of any of claims 1-4, wherein: the preparation process comprises the following steps: