CN110673353B - Super-resolution focusing device based on high-refractive-index material - Google Patents
Super-resolution focusing device based on high-refractive-index material Download PDFInfo
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- CN110673353B CN110673353B CN201911104030.6A CN201911104030A CN110673353B CN 110673353 B CN110673353 B CN 110673353B CN 201911104030 A CN201911104030 A CN 201911104030A CN 110673353 B CN110673353 B CN 110673353B
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/58—Optics for apodization or superresolution; Optical synthetic aperture systems
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B3/00—Simple or compound lenses
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- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
- G03F7/70—Microphotolithographic exposure; Apparatus therefor
- G03F7/70216—Mask projection systems
- G03F7/70325—Resolution enhancement techniques not otherwise provided for, e.g. darkfield imaging, interfering beams, spatial frequency multiplication, nearfield lenses or solid immersion lenses
Abstract
The invention provides a super-resolution focusing device based on a high-refractive-index material. The device comprises a light source, a focusing element and a similar lens focusing structure, wherein light beams emitted by the light source are primarily focused into focusing light or parallel light through the focusing element and then enter the similar lens focusing structure for further focusing, and a focusing light spot with a sub-wavelength size is formed on a focusing surface of the similar lens focusing structure; the material of the lens-like focusing structure adopts a material with a refractive index larger than 3. The device can reduce the focusing light spot to lambda/10, can further improve the focal depth of the focusing light spot by utilizing a radial polarized light illumination mode, can keep the size of the focusing light spot to lambda/8, can reach the focal depth of lambda/5, and is expected to be used in the fields of nano-scale super-resolution lithography, imaging, data storage and the like.
Description
Technical Field
The invention belongs to the field of laser beam focusing photoetching and imaging, and particularly relates to a super-resolution focusing device based on a high-refractive-index material.
Background
The nanolithography technique has been widely used in the processing field due to its advantages of large area, good repeatability, and easy operation. The processing of the nano-scale pattern cannot be directly realized by adopting the conventional optical exposure process due to the limitation of the light diffraction limit delta x being lambda/2 NA. In order to improve the processing precision and improve the diffraction limit, the method for improving the resolution mainly comprises two methods:
the first is to reduce the wavelength of light used for optical exposure, and the most widely used at present are deep ultraviolet exposure (DUV) with wavelengths of 248nm and 193nm, and extreme ultraviolet Exposure (EUV) with a wavelength of 13.5 nm. The technology has complicated light path design, extremely expensive light source and equipment and complicated maintenance.
The second is to increase the numerical aperture, which can effectively reduce the size of the light spot and increase the resolution by using a high numerical control immersion lens, for example, the numerical aperture of the objective lens can be increased to about 1.5 by using an oil immersion (oil lens) or water immersion (water lens). However, the refractive index of the material of such solid immersion lenses is below 2, which greatly limits the focus resolving power of such methods.
At present, no immersion lens with the working waveband in the ultraviolet or near ultraviolet waveband is manufactured by directly using a material with the refractive index larger than 3. In materials science, the absorption loss of the material with high refractive index is necessarily large. The refractive indexes of common high-refractive-index materials such as Si, Ge and GaAs are far more than 3 in a visible light waveband, but the materials have very strong absorption in the visible light waveband and are opaque in a conventional micron size, so that a solid immersion lens made of the materials is mostly used in an infrared waveband and has little meaning for photoetching. Meanwhile, because of the extremely strong loss, and the refractive index and the absorption rate of the material are changed along with the wavelength of the light wave, the material is greatly different from the traditional transparent material in the aspect of focus analysis, and the influence of the imaginary part of the refractive index needs to be additionally considered. Therefore, the focused spot size of such high index materials and the structural dimensions of the lens are closely related to the wavelength of the light wave.
However, an indirect bandgap material is a material that absorbs less strongly, such as Si. The penetration depth of the material is smaller, which is often smaller than a wavelength, namely, submicron level, and the numerical aperture can be improved by utilizing the extremely high refractive index of the material by utilizing the method of reducing the optical path, namely, reducing the size of the focusing lens to the submicron level, so that the resolution capability is improved, and the effect of super resolution is achieved. Lenses of conventional materials are inoperative at this scale due to the presence of near field effects, where classical geometric optics are no longer applicable and need to be explained using scattering theory, and the resulting focused spot is generated by resonant interference of the beam in the structure of the high index focusing lens. Meanwhile, the penetration depth of the material is directly related to the imaginary part of the refractive index of the material, the incident beam is partially attenuated in the lens structure and then focused, and the size of a focused light spot is determined by the size and the shape of the designed lens structure, the wavelength, the polarization and the incident direction of the incident beam.
Disclosure of Invention
In view of the above prior art, the present invention aims to provide a super-resolution focusing device based on high refractive index material to improve laser beam focusing and imaging capabilities.
In order to achieve the purpose, the invention adopts the technical scheme that:
the super-resolution focusing device based on the high-refractive-index material comprises a light source, a focusing element and a similar lens focusing structure, wherein light beams emitted by the light source are primarily focused into focusing light or parallel light through the focusing element and then enter the similar lens focusing structure for further focusing, and a focusing light spot with a sub-wavelength size is formed on a focusing surface of the similar lens focusing structure; the material of the lens-like focusing structure is a material with a refractive index larger than 3.
Further, the light beam is natural light, circularly polarized light, elliptically polarized light, partially polarized light, linearly polarized light, radially polarized light, or azimuthally polarized light.
Further, the wavelength range of the light beam is 350nm to 2000 nm.
Further, the material of the lens-like focusing structure adopts a material with a refractive index larger than 3 and an imaginary part of the refractive index not larger than 1.
Further, the material of the lens-like focusing structure is silicon, germanium or a silicon-germanium alloy.
Further, the lens-like focusing structure is a hemispherical structure, a hyper-hemispherical structure, a spherical structure or a modified structure of the above structures, or is a hyper-lens structure.
Further, the full width at half maximum of the focusing light spot is less than 100 nm.
Furthermore, a covering layer is arranged around the lens-like focusing structure, and the edge of the lens-like focusing structure is overlapped with the edge of the covering layer.
The traditional method for improving the NA is a solid immersion method, the selected material is a transparent material, but the refractive index does not exceed 3, and therefore the focusing size is limited; in addition, conventional designs are above micron in size, such as microspheres, or microlenses. To solve the refractive index problem, a narrow bandgap semiconductor with an ultra-high refractive index can be used, but this introduces an absorption problem. The invention adopts two solutions simultaneously: firstly, an indirect band gap material is used, and the absorption is weaker than that of a direct band gap material; and secondly, reducing the optical path, namely reducing the size of the lens, and designing the lens by utilizing the wave optics principle. By the technical means, the refractive index of more than 3 and higher light passing rate can be obtained simultaneously in ultraviolet and visible light, so that the numerical aperture is improved to advance the diffraction limit, and high-brightness super-resolution focusing with submicron size is obtained.
Compared with the prior art, the invention has the advantages that:
(1) the invention has simple optical path, does not need complex optical path design and harsh preparation environment;
(2) the invention has simple structure, easy design, easy acquisition and preparation of materials;
(3) the light source of the invention has low cost and does not need excessive structural modification;
(4) the device can obtain a focused light spot with the size of 40nm, and the focal depth of the radial polarized light beam can reach 75 nm;
(5) the device of the invention can be applied to super-resolution imaging, nano-lithography, precision machining and other purposes.
Drawings
FIG. 1 is a structural diagram of a super-resolution focusing apparatus according to an embodiment of the present invention;
FIG. 2 is a graph of the intensity distribution in the XY plane of the device of the present invention using linearly polarized light to focus light;
FIG. 3 is a diagram showing the ratio of the light intensity distribution in the X-axis direction and the Y-axis direction of the light spot focused by the linearly polarized light according to the apparatus of the present invention;
FIG. 4 is a graph of the intensity distribution in the XY plane of the device of the present invention using radially polarized light focusing;
FIG. 5 is a schematic diagram showing the comparison of the intensity distribution in the Z-axis direction between the linearly polarized focused light spot and the radially polarized focused light spot;
FIG. 6 is a graph showing a comparison of the intensity distribution of a linearly polarized focused spot with the addition of a cover layer to the device of the present invention;
fig. 7 is a graph of spot sizes at different wavelengths for a lens-like structure having a hemispherical radius of 150 nm.
Detailed Description
The general technical idea of the invention is as follows: generating a working beam by a laser; manufacturing a high-refractive-index material into a hundred-nanometer-size lens-like structure capable of focusing, and placing the hundred-nanometer-size lens-like structure at the optimal position of a working light beam; focusing the primarily focused focusing light spots again through a high-refractive-index lens structure; super-resolution focusing light spots with the half-height width smaller than 100nm can be obtained at the emergent end of the high-refractive-index lens structure.
In the following, the high refractive index material is silicon, the lens-like structure is a hemisphere, and the light source is a high-pressure mercury lamp H-line (405nm), but the invention is not limited thereto (it can be used for ultraviolet and visible light).
As shown in fig. 1, an apparatus for implementing super-resolution focusing sequentially includes: a light source 1, a microscope objective 2 and a hundred nanometer silicon hemisphere 3. The hundred-nanometer silicon hemisphere 3 and the microscope 2 are both located on a light path of an emergent light beam of the light source 1, the size of the hundred-nanometer silicon hemisphere needs to be determined according to the wavelength and the polarization of the incident working light beam and the numerical aperture of the microscope 2, and the size range can be 50-400 nm. Linearly polarized light beams generated by an H line (405nm) of a high-pressure mercury lamp are used for forming focused light beams after primary convergence of the microscope objective lens 2, the focused light beams irradiate the hundred-nanometer silicon hemisphere 3 placed at the focal point of the microscope objective lens 2 for further focusing, and an ultra-small focused light spot can be obtained at the section emergent end of the hundred-nanometer silicon hemisphere 3.
The refractive index of silicon of the high-refractive-index material is 5.42 under a light wave of 405nm, the resolving power can be improved by 5.42 times according to the diffraction limit formula of delta x ═ lambda/2 nsin theta, but the silicon has strong absorption in a visible light wave band and is opaque in a conventional size. However, silicon is an indirect bandgap material with a small imaginary part in the visible band and a penetration depth of around a hundred nanometers. Therefore, silicon can be scaled down to the order of hundreds of nanometers, taking advantage of the high refractive index of silicon materials to increase the numerical aperture without considering the interference of its absorption. For hundreds of nano structures, the focusing size of the traditional geometric optics is not applicable, and the formation of the focusing light spot is caused by the superposition of optical wave resonance in a silicon hemisphere cavity, so the size of the focusing light spot is limited by the size of the silicon hemisphere and the wavelength and polarization of incident light.
The light intensity distribution schematic diagram of the focusing light spot in the XY plane is shown in FIG. 2, the full-peak half-height width in the X direction can reach 42nm and the full-peak half-height width in the Y direction can reach 44nm, and the size of the focusing light spot is lambda/10, which is improved by more than 4 times compared with the resolution of 0.414 lambda of a ball lens made of a traditional material (for example, Chinese patent CN 102226855A), and the focusing light spot can be used for irradiating, processing or detecting a sample.
The working light beam can be natural polarized light, circular polarized light, elliptical polarized light, partial polarized light, linear polarized light, radial polarized light and angular polarized light. In order to improve the focal depth, a radial polarized light beam is adopted as a working light beam, namely a light source 1 adopts a high-pressure mercury lamp H-line (405nm) laser light source and a polarization converter to be combined, the radial polarized light beam is emitted and is primarily converged by a microscope objective lens 2, a focusing light spot is obtained at the emergent end of the section of the hundred-nanometer hemisphere 3, the light intensity distribution schematic diagram of the focusing light spot in an XY plane is shown in figure 4, the transverse resolution capability of the focusing light spot is 60nm, and the size of the focusing light spot reaches lambda/8. The light intensity distribution of the linear polarized light beam in the Z-axis direction is shown in FIG. 5, the focal depth of the radial polarized focusing light spot on the Z-axis can reach 75nm, and the focal depth reaches lambda/5.
A cover layer with high absorption properties can be added around the hundred nm silicon hemisphere 3 to suppress side lobes 4 (see fig. 1). The covering layer 4 can be made of a high-absorption metal material such as aluminum, the position of the covering layer 4 is just overlapped with the edge of the hundred-nanometer silicon hemisphere structure 3, the hundred-nanometer silicon hemisphere 3 is just embedded into the covering layer 4, the thickness of the covering layer 4 is related to the size of the hundred-nanometer silicon hemisphere 3, the addition of the high-absorption material covering layer 4 can well inhibit sharp-angle side lobes caused by the hemisphere structure, namely after a working light beam is focused through the hundred-nanometer hemisphere 3, a focused light spot with side lobes removed can be obtained through the absorption effect of the high-absorption material covering layer 4, and the light intensity distribution schematic diagram of the focused light spot formed by the linear polarization light beam in the XY plane is shown in fig. 6.
The linear polarization light beam can be changed into a radial polarization light beam or an angular polarization light beam, and the hundred-nanometer silicon hemisphere 3 can be directly irradiated by the working light beam without adopting the microscope objective 3. The wavelength of the light source 1 may be 405nm, or 380nm, or may be any wavelength in the visible light band, or may be any wavelength in the near ultraviolet band, and it is necessary to combine with the size of the lens-like focusing structure as shown in fig. 7.
The high refractive index material silicon of this embodiment may also be replaced with a silicon germanium alloy, or a material having a high refractive index and a low absorption rate. The lens-like focusing structure can be a hemisphere and a modification structure thereof, can also be a sphere and a modification structure thereof, can also be an ultrahemisphere and a modification structure thereof, can also be a focusing structure modified by a certain radian, and can also be an ultralens structure capable of focusing the working light beam. The size of the lens-like focusing structure is directly related to the wavelength sum of the incident light. The thickness is less than 1 micron and may be slightly larger than the wavelength of the incident light. The wavelength of the adopted light source can be a visible light band or a near ultraviolet band, and the wavelength which meets the penetration requirement for the highest refractive index and the imaginary part of the material is preferred.
Claims (6)
1. The super-resolution focusing device based on the high-refractive-index material comprises a light source, a focusing element and a similar lens focusing structure, and is characterized in that light beams emitted by the light source are primarily focused into focusing light or parallel light through the focusing element and then enter the similar lens focusing structure for further focusing, and a focusing light spot with a sub-wavelength size is formed on a focusing surface of the similar lens focusing structure; the material of the lens focusing structure adopts silicon, germanium or silicon-germanium alloy, and the size range of the lens focusing structure is 50-400 nm.
2. The super-resolution focusing device based on high refractive index material according to claim 1, wherein the light beam is natural light, circularly polarized light, elliptically polarized light, partially polarized light, linearly polarized light, radially polarized light, or azimuthally polarized light.
3. The super resolution focusing device based on high refractive index material according to claim 1, wherein the wavelength range of the light beam is 350nm to 2000 nm.
4. The super-resolution focusing device based on high refractive index material as claimed in claim 1, wherein the lens-like focusing structure is a hemispherical structure, a hyper-hemispherical structure, a spherical structure or a modified structure thereof, or a hyper-lens structure.
5. The super-resolution focusing device based on high refractive index material as claimed in claim 1, wherein the half width of the focused light spot is less than 100 nm.
6. The super-resolution focusing device based on high refractive index material as claimed in claim 1, wherein a cover layer is further disposed around the lens-like focusing structure, and the edge of the lens-like focusing structure coincides with the edge of the cover layer.
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| JP2003107358A (en) * | 2001-09-27 | 2003-04-09 | Fujitsu Ltd | Optical device using solid immersion lens |
| US20030067860A1 (en) * | 2001-10-10 | 2003-04-10 | Industrial Technology Research Institute | Near-field optical flying head |
| CN1332387C (en) * | 2005-03-09 | 2007-08-15 | 浙江大学 | Solid immersion lens for near field optical storage |
| CN102226855B (en) * | 2011-05-26 | 2012-11-14 | 浙江大学 | Three-dimensional super-resolution focusing method and device based on transparent medium pellet |
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| 《A New Dielectric Metamaterial Building Block wit a Strong Magnetic Response in the Sub-1.5-Micrometer Region:Silicon Colloid Nanocavities》;LEI Shi 等;《ADVANCED MATERIALS》;20121231(第24期);第5934-5938页 * |
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