Ultra-long focal depth super-resolution optical needle generation method and high aspect ratio laser direct writing system

Abstract

The present invention relates to a method for generating a long focal depth super-resolution light needle and a high aspect ratio laser direct writing system. The method adopts the characteristic of multi-focal point splicing to extend the focal depth, combines with a genetic algorithm to break the contradiction between the numerical aperture and focal depth of a traditional lens, and regulates the incident light field through a super-oscillating lens to realize a long focal depth light field under a high numerical aperture, thereby obtaining an ultra-long focal depth super-resolution light needle light field that exceeds the diffraction limit. Ultimately, the ultra-long focal depth super-resolution light needle light field that exceeds the diffraction limit is used to realize high aspect ratio laser direct writing processing, thereby realizing the processing of micro-nano structures with a line width less than the diffraction limit and with certain processing depth requirements. In addition, the ultra-long focal depth super-resolution light needle light field generated by the present invention can be directly acquired in real time without any bulky and complex mechanical structure, thereby making the architecture of the high aspect ratio laser direct writing system simpler and facilitating the miniaturization design of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of high aspect ratio micro-nanostructure processing, and in particular to a method for generating an ultra-long focal depth super-resolution light needle and a high aspect ratio laser direct writing system thereof. Background Art

[0002] With the rapid iteration of micro-nano technology and micro-nano devices, many industries including semiconductors have increasingly stringent requirements for micro-nano processing technology; in the field of micro-nano manufacturing, compared with point scanning micro-processing manufacturing technologies such as electron beam etching technology, ion beam etching technology, and near-field scanning writing technology, laser direct writing technology is a flexible and low-cost processing technology. Due to its high efficiency and high precision, it is widely used in diffraction elements, flexible electronic devices, microfluidic chip manufacturing, etc.; laser direct writing technology is a maskless lithography technology that uses laser to achieve direct writing. It scans a substrate with a photosensitive layer through a laser beam to directly produce the required structure without preparing a mask plate. At the same time, it can omit the processes of graphic transfer and overlay, and has the characteristics of strong flexibility and low cost.

[0003] High aspect ratio micro-nanostructures have shown great potential application value in the fields of biosensors, three-dimensional micro batteries, photonic crystals, and three-dimensional integrated circuit interconnections. Laser direct writing technology has become an important way to prepare high aspect ratio micro-nanostructures due to its processing diversity and flexibility. Therefore, how to achieve the processing and preparation of high aspect ratio micro-nanostructures is crucial; however, due to the existence of the diffraction limit, the focused light spot of the far-field optical system cannot be focused to an ideal point, which results in the resolution of a series of application technologies based on the far-field optical system being limited to half the wavelength, including the laser direct writing system; after continuous development, nanoimprint lithography Lithography technologies such as technology, projection lithography technology, and laser direct writing technology have been widely used in the preparation of micro-nano structures; however, in laser direct writing technology, the beam quality of the direct writing spot directly determines the structure that can be prepared; but in the focusing system, the focused spot size and focal depth are mutually constrained, and it is difficult to meet the requirements of subwavelength size and extremely long focal depth at the same time; therefore, it is difficult to prepare micro-nano structures with high aspect ratio using traditional focused spots; how to break through the limitations of traditional theory and design and obtain a light needle structure focused spot that can have both high resolution and long focal depth has become an urgent problem to be solved in the direct writing processing of micro-nano structures with high aspect ratio. Summary of the Invention

[0004] The problem solved by the present invention is how to design a light needle with both super-resolution and long focal depth, and to design a high-aspect ratio laser direct writing system based on the designed long-focal-depth super-resolution light needle to prepare a high-aspect ratio micro-nano structure.

[0005] To solve the above problems, the present invention provides a method for generating ultra-long focal depth super-resolution light needles, comprising:

[0006] Step 1: Generate N identical high-numerical-aperture phase masks using a random number generator, and calculate the N light fields generated by the N high-numerical-aperture phase masks based on vector angular spectrum theory. The N high-numerical-aperture phase masks serve as the initial population, and each light field has M focal points distributed on the optical axis.

[0007] Step 2: Set the optimization goal, give constraints based on the optimization goal, and set the requirements for focus stitching on the optical axis of the light field on the rear surface of the lens. Stitch the M focuses behind the high numerical aperture phase mask to form a long focal depth light needle with uniform intensity and a half-height width exceeding the diffraction limit;

[0008] Step 3: Use the N high numerical aperture phase masks in the initial population as chromosomes X=(X1, X2, ...X i ,...X N );X i represents the i-th high numerical aperture phase mask, and the initial genetic generation k = 1;

[0009] Step 4: Calculate the fitness function value of each chromosome;

[0010] Step 5: Perform crossover and mutation operations on the chromosomes in the current population, and superimpose the constraints set in step 2 as penalty function terms into the optimization objective. Use the penalty function to modify the constraints of each chromosome in the current population; generate the next generation population;

[0011] Step 6: Determine whether k reaches the preset maximum genetic generation number K. If so, terminate the optimization and go to step 7. Otherwise, k = k + 1, and use the next generation population as the chromosome in the genetic algorithm population, and return to step 4.

[0012] Step 7: Select the chromosome with the highest fitness function value from the current population, thereby obtaining an ultra-long focal depth super-resolution light needle corresponding to a high numerical aperture phase mask.

[0013] The beneficial effects of the ultra-long focal depth super-resolution light needle generation method of the present invention are: using the characteristics of multi-focal point splicing to extend the focal depth, combining with genetic algorithms to break through the contradiction between the numerical aperture and focal depth of traditional lenses, and regulating the incident light field through a super-oscillation lens to achieve a long focal depth light field under high numerical aperture, and obtain an ultra-long focal depth super-resolution light needle light field that exceeds the diffraction limit; in addition, the ultra-long focal depth super-resolution light needle light field generated by the present invention does not require any bulky and complex mechanical structure in space and can be directly acquired in real time, with a simpler system, more conducive to the rapid real-time capture of information and the miniaturization design of the system.

[0014] Preferably, the optimization goal set in step 2 is: the intensity uniformity of the focus in the light field is consistent, and the half-width height of the focus is smaller than the diffraction limit;

[0015] The focus stitching constraint conditions given in step 2 are: the maximum intensity points of adjacent focuses overlap continuously, and the half-width heights of adjacent focuses are consistent.

[0016] Preferably, the step 4 of calculating the fitness function value in each chromosome specifically includes: given the focal length and incident wavelength, using the vector angular spectrum theory to calculate the objective function value corresponding to the high numerical aperture phase mask, and using the calculated objective function value as the fitness function value.

[0017] A high aspect ratio laser direct writing system based on the ultra-long focal depth super-resolution optical needle, comprising:

[0018] Laser, used to provide a laser light source with a preset wavelength for laser direct writing processing;

[0019] A beam expander, used to expand the incident laser light source to a preset multiple;

[0020] A polarizer is used to modulate the polarization state of the expanded laser light source to form a polarized beam;

[0021] A super-oscillating lens is used to control the modulated polarized light beam to generate multiple focal points at a preset focal length to extend the focal depth of the super-oscillating lens and obtain a super-resolution light needle light field distribution with a focal size smaller than the diffraction limit;

[0022] The exposure platform is used to place the photoresist to be laser etched. The super-resolution light needle generated by the super-oscillation lens exposes the photoresist according to a preset path to obtain a high aspect ratio super-resolution micro-nano structure.

[0023] The beneficial effects of the high aspect ratio laser direct writing system of the present invention are: based on the ultra-long focal depth super-resolution light needle generated by the super-oscillating lens, it breaks through the contradiction between numerical aperture and focal depth in the traditional focusing system, has a smaller focal size, and realizes super-resolution, high aspect ratio laser direct writing lithography processing that is not affected by traditional Bessel beams.

[0024] Preferably, the material of the super-oscillation lens is fused quartz or calcium fluoride. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a flow chart of specific embodiment 1 of the present invention;

[0026] Figure 2 (a) Schematic diagram of the super-oscillating lens and its focusing principle according to the first embodiment of the present invention;

[0027] Figure 2(b) A side view of the focusing principle of the super-oscillating lens according to the first embodiment of the present invention;

[0028] Figure 2 (c) Schematic diagram of extending the focal depth by splicing multiple focal points of a super-oscillating lens according to specific embodiment 1 of the present invention;

[0029] Figure 3 (a) Schematic diagram of the system structure of specific embodiment 2 of the present invention;

[0030] Figure 3 (b) Schematic diagram of the interaction between the light needle generated by the super-oscillating lens and the structure to be directly written;

[0031] Figure 4 This is a diagram illustrating the steps of triggering the laser direct writing scanning pattern in the workflow of specific embodiment 2 of the present invention.

[0032] Description of reference numerals:

[0033] 1. Laser; 2. Beam expander; 3. Polarizer; 4. Super-oscillating lens; 5. Photoresist; 6. Exposure platform. DETAILED DESCRIPTION

[0034] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. Specific embodiment 1

[0036] like Figure 1 A method for generating a super-resolution light needle with an ultra-long focal depth is shown, comprising:

[0037] Step 1: Generate N identical high-numerical-aperture phase masks using a random number generator, and calculate the corresponding N light field distributions generated by each of the N high-numerical-aperture phase masks based on vector angular spectrum theory. The N high-numerical-aperture phase masks serve as the initial population, and each light field has M focal points distributed on the optical axis.

[0038] Step 2: Set the optimization goal: the intensity uniformity of the focus in the light field is consistent, the half-width height of the focus is less than the diffraction limit, and the constraints are given according to the optimization goal; and the focus stitching constraints are given: the maximum intensity points of adjacent focuses are continuously overlapped, and the half-width height of adjacent focuses is consistent;

[0039] Step 3: Use the N high numerical aperture phase masks in the initial population as chromosomes X=(X1, X2, ...X i ,...X N );X i represents the i-th high numerical aperture phase mask;

[0040] Step 4: Calculate the fitness function value in each chromosome, specifically including: given the focal length and incident wavelength, use the vector angular spectrum theory to calculate the target function value corresponding to the high numerical aperture phase mask, and use the calculated target function value as the fitness function value;

[0041] Step 5: Perform crossover and mutation operations on the chromosomes in the current population, and superimpose the constraints set in step 2 as penalty function terms into the optimization objective. Use the penalty function to constrain and modify the chromosomes in the current population to generate the next generation population.

[0042] Step 6: Determine whether k reaches the preset maximum genetic generation number K. If so, terminate the optimization and go to step 7. Otherwise, k = k + 1, and use the next generation population as the chromosome in the genetic algorithm population, and return to step 4.

[0043] Step 7: Select the chromosome with the highest fitness function value from the current population, thereby obtaining an ultra-long focal depth super-resolution light needle corresponding to a high numerical aperture phase mask;

[0044] In the present invention, the super-oscillating lens is a plane lens that uses a ring-shaped structure to achieve fine and complex control of the incident plane wave. The focusing principle diagram is shown in Figure 2 (a), wherein the purpose of extending the depth of focus is achieved by stitching multiple focal points generated by the super-oscillating lens, the depth of focus size DOF; see Figure 2 (b) The ring structure of the super-oscillating lens has a substrate thickness of h1 and a structural layer thickness of h2; the super-resolution light needle generated by the super-oscillating lens has a length of DOF and a half-height width smaller than the diffraction limit; see Figure 2 (b) The fabrication resolution of the super-oscillating lens laser direct writing system is determined by the focal plane half-maximum width of the super-resolution light needle; see Figure 2 (c) The light field after the super-oscillation lens is emitted realizes a long focal depth DOF light field at a predetermined focal length, and the half-height width of the focus exceeds the diffraction limit; in this specific embodiment, Figure 2 (b) Concentric ring structure with super-oscillation lens radius of 500μm, substrate h1 = 500μm, and structural layer thickness h2 = 215nm. The super-resolution light needle generated by the super-oscillation lens has DOF ​​= 6μm and half-height width of 130nm.

[0045] Therefore, the present invention adopts the characteristics of multi-focal point splicing to extend the focal depth, combines with genetic algorithms to break the contradiction between the numerical aperture and focal depth of traditional lenses, and regulates the incident light field through the super-oscillation lens to achieve a long focal depth light field under high numerical aperture, and obtain an ultra-long focal depth super-resolution light needle light field that exceeds the diffraction limit; in addition, the ultra-long focal depth super-resolution light needle light field generated by the present invention does not require any bulky and complex mechanical structure in space and can be directly acquired in real time, with a simpler system, more conducive to the rapid real-time capture of information and the miniaturization design of the system. Specific embodiment 2

[0047] like Figure 3 A high aspect ratio laser direct writing system based on the ultra-long focal depth super-resolution light needle generation method of specific embodiment 1 shown in (a) includes the following arranged in sequence:

[0048] Laser 1, used to provide a laser light source with a preset wavelength for laser direct writing processing;

[0049] Beam expander 2, used to expand the incident laser light source to a preset magnification;

[0050] Polarizer 3, used to modulate the polarization state of the laser light source after beam expansion to form a polarized beam;

[0051] The super-oscillation lens 4 is used to control the modulated polarized light beam to generate multiple focal points at a preset focal length to extend the focal depth of the super-oscillation lens 4 and obtain a super-resolution light needle light field distribution with a focal size smaller than the diffraction limit; the material of the super-oscillation lens 4 is fused quartz or calcium fluoride; the exposure platform 6 is used to place the photoresist 5 to be laser etched, and the super-resolution light needle generated by the super-oscillation lens 4 exposes the photoresist 5 according to a preset path to obtain a high aspect ratio super-resolution micro-nano structure.

[0052] In this specific embodiment, the super-oscillation lens 4 has an operating wavelength of 405 nm and is made on a 500 μm thick glass substrate. The structural layer is a Si3N4 material with a refractive index of 2.25 and a thickness of 215 nm. When the laser emitted by the laser 1 is expanded by the beam expander 2, it is converted from a parallel beam into a polarized beam through the polarizer 3, and then passes through the super-oscillation lens 4 to generate a beam of super-resolution high-depth and wide light needle, which is finally irradiated onto the exposure platform 6. The photoresist 5 is located in the exposure platform 6, and the upper computer is used to control the movement of the exposure platform 6, so that the high-depth and wide super-resolution light needle generated by the super-oscillation lens 4 is scanned according to a pre-set path to expose the photoresist 5 to obtain a high-aspect ratio super-resolution micro-nano structure.

[0053] like Figure 3 As shown in (b), a high aspect ratio super-resolution light needle generated by a super-oscillating lens 4 is used as a writing light source to irradiate the photoresist 5, and the photoresist 5 is exposed by utilizing its point-by-point scanning characteristics. Since the light beam has the characteristics of super-resolution and large depth of focus, it can perform point-by-point exposure of a certain depth and super-resolution size.

[0054] Workflow such as Figure 4 As shown:

[0055] Step 1: Turn on the processing femtosecond laser 1, and direct the incident light source 405nm laser 1 through the collimating lens to the super-oscillation lens 4 to generate a super-resolution light needle with a length of 6μm;

[0056] Step 2: Use the host computer to control the movement of the exposure platform 6, so that the light needle generated by the super-oscillation lens 4 is processed according to a micro-nano pinhole array with a period of 1 μm and an aperture of 130 nm, and a total of 20×20 rows of micro-nano pinholes are processed to achieve relative movement between the laser beam and the direct-writing material 1 in the axial direction. Place a substrate of a photoresist 5 with a uniform thickness of 5 μm on the sample platform, set the laser incident power, and make the light generated by the super-oscillation lens 4 expose the photoresist 5 point by point according to the set pattern;

[0057] Step 3: Develop the exposed structure substrate using a developer corresponding to the photoresist 5;

[0058] Step 4: After a period of time, the exposed pattern deteriorates and dissolves in the developer, and the laser-written structure is revealed. The developer on the surface of the structure is washed away with deionized water to obtain a periodic micro-nano pinhole structure.

[0059] Although the present disclosure is disclosed as above, the protection scope of the present disclosure is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present disclosure, and these changes and modifications will fall within the protection scope of the present invention.

Claims

1. A method for generating ultra-long focal depth super-resolution light needles, characterized in that: include: Step 1: Generate N identical high-numerical-aperture phase masks using a random number generator, and calculate the N light fields generated by the N high-numerical-aperture phase masks based on vector angular spectrum theory. The N high-numerical-aperture phase masks serve as the initial population, and each light field has M focal points distributed on the optical axis. Step 2: Set the optimization goal, give constraints based on the optimization goal, and set the requirements for focus stitching on the optical axis of the light field. Stitch the M focuses behind the high numerical aperture phase mask to form a long focal depth light needle with uniform intensity and a half-height width exceeding the diffraction limit. Step 3: Use the N high numerical aperture phase masks in the initial population as chromosomes X=(X1, X2, ...X i ,...X N );X i represents the i-th high numerical aperture phase mask, and the initial genetic generation k=1; Step 4: Calculate the fitness function value of each chromosome; Step 5: Perform crossover and mutation operations on the chromosomes in the current population, and superimpose the constraints set in step 2 as penalty function terms into the optimization objective. Use the penalty function to modify the constraints of each chromosome in the current population; generate the next generation population; Step 6: Determine whether k reaches the preset maximum genetic generation number K. If so, terminate the optimization and go to step 7. Otherwise, k=k+1, and use the next generation population as the chromosome in the genetic algorithm population, and return to step 4. Step 7: Select the chromosome with the highest fitness function value from the current population, thereby obtaining an ultra-long focal depth super-resolution light needle corresponding to a high numerical aperture phase mask.

2. The method for generating ultra-long focal depth super-resolution light needles according to claim 1, characterized in that: The optimization goal set in step 2 is: the intensity of each focus on the optical axis is uniform and the half-width height of each focus is less than the diffraction limit; The focus stitching constraint conditions given in step 2 are: the maximum intensity points of adjacent focuses overlap continuously, and the half-width heights of adjacent focuses are consistent.

3. The method for generating ultra-long focal depth super-resolution light needles according to claim 2, wherein: Calculating the fitness function value in each chromosome in step 4 specifically includes: given the focal length and incident wavelength, using vector angular spectrum theory to calculate the light field distribution behind each random phase mask, and using the calculated light field distribution as the fitness function value of each random phase mask.

4. A high aspect ratio laser direct writing system based on the ultra-long focal depth super-resolution optical needle generation method according to any one of claims 1 to 3, characterized in that: include: A laser (1) for providing a laser light source with a preset laser direct writing incident wavelength; A beam expander (2) is used to expand the incident laser light source to a preset magnification; A polarizer (3) is used to modulate the polarization state of the laser light source after beam expansion to form a polarized light beam; A super-oscillating lens (4) is used to regulate the modulated polarized light beam to generate multiple focal points at a preset focal length to extend the focal depth of the super-oscillating lens (4) and obtain a super-resolution light needle light field distribution with a focal size smaller than the diffraction limit; An exposure platform (6) is used for placing a photoresist (5) to be laser-etched. The super-resolution light needle generated by the super-oscillation lens (4) exposes the photoresist (5) according to a preset path to obtain a high aspect ratio super-resolution micro-nano structure.

5. The high aspect ratio laser direct writing system according to claim 4, characterized in that: The material of the super-oscillation lens (4) is fused quartz or calcium fluoride.

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