A method for producing ordered subwavelength nanostripes using femtosecond laser

Abstract

The present invention discloses a method for producing ordered subwavelength nanostripes using a femtosecond laser. The method comprises: irradiating a composite film composed of a metal and a semiconductor with a single femtosecond laser beam; the metal film is formed by evaporation or sputtering of a high-loss material, and the semiconductor film is composed of a material with high optical absorption and susceptible to oxidation. Under the action of the femtosecond laser, quasi-cylindrical waves are excited on the surface of the composite film, which interfere with the incident laser light to form periodically distributed oxidation stripes; the stripes are oriented perpendicular to the polarization direction of the incident femtosecond laser light.

Description

Technical Field

[0001] The present invention belongs to the field of laser advanced micro-nano manufacturing, and specifically relates to a manufacturing method for forming a periodic sub-wavelength oxidation stripe structure on the surface of a metal-semiconductor composite film by self-organization using femtosecond laser irradiation. Background Art

[0002] Images composed of subwavelength periodic textures can, through the principles of optical diffraction, display richer and more diverse colors as the viewing angle changes, thus possessing considerable artistic and anti-counterfeiting value. Currently, mature technologies widely used to fabricate periodic nanostructures include electron beam lithography, focused ion beam etching, nanoimprinting, laser direct writing, laser interferometry, and chemical synthesis. However, these technologies rely on expensive and complex operating systems, specialized materials, inefficient preparation methods that preclude large-scale processing, or low controllability of the processing process. Therefore, developing methods for rapidly fabricating, large-scale, low-cost, controllable, and high-quality subwavelength periodic surface textures is of great scientific significance and industrial application value.

[0003] Single-beam laser-induced self-organized periodic surface structures offer a potential solution to these challenges. Although research on laser-induced self-organized stripe structures has spanned over half a century, this method has yet to be effectively promoted as a micro-nanofabrication technology. The fundamental reason is that the laser-induced self-organization process is uncontrollable, resulting in long-range disorder and a high degree of randomness in the resulting stripe structure. Several solutions have been proposed to address this issue. For example, femtosecond lasers can be used to induce thermochemical reactions to produce oxide particle accumulation. The formation of this stripe structure involves a nonlocal feedback effect, which can improve the regularity of the self-organized periodic structure to a certain extent. Another example is the "strong ablation" effect induced by high-energy femtosecond lasers, which rapidly vaporizes and evaporates surface materials, reducing surface debris. Alternatively, femtosecond laser pulse spatiotemporal shaping can be used to reduce the interference of surface debris on surface electromagnetic waves through ablation cooling, thereby improving the regularity of the periodic stripe structure. Another example is the use of high-optical-loss metal materials (such as Ti, Cr, and Mo) to reduce the decay length of surface plasmons and thus enhance the coherence of surface electromagnetic waves.

[0004] All of the above methods can improve the regularity of the self-organized stripe structure to a certain extent, but they all have a prerequisite, which is the need to use a small spot (spot diameter is generally <10 wavelengths) for point-by-point scanning. This is because when a large spot is used, there are a large number of random surface defects in the spot illumination area. These defects will act as random "seeds", causing the self-organized stripe structure to bifurcate and become disordered. Although using a small spot scan can effectively reduce the number of random "seeds" in the steady-state exposure area and improve the regularity of the periodic stripe structure, it does not guarantee a very collimated periodic stripe structure. This is because as the spot moves, new random "seed" structures will continue to appear at the center of the spot. These subsequent "random seeds" may be slightly misaligned in space with the previously formed stripes, causing the final periodic stripes to bend. In addition, the above methods are all based on the interference effect of laser and surface plasmon on a single material. Summary of the Invention

[0005] The present invention provides a method for manufacturing ordered sub-wavelength nano-stripes using a femtosecond laser, which does not require the separate setting of "induced seeds" and can directly obtain periodic oxidation stripes.

[0006] A method for manufacturing ordered subwavelength nanostripes using a femtosecond laser comprises: irradiating the semiconductor film side of a metal-semiconductor composite film with a femtosecond laser to induce an oxidation reaction in the semiconductor material to form oxide particles, further forming oxide nanorods under the action of a near-field enhancement effect, and finally self-organizing to form periodically distributed oxide stripes under the induction of a quasi-cylindrical wave excited by the laser.

[0007] The present invention relates to a method for inducing self-organization and generating periodic oxidation stripes on the surface of a metal-semiconductor composite thin film system using steady-state irradiation with a single femtosecond laser beam. Specifically, the method utilizes the quasi-cylindrical waves excited by the femtosecond laser within the composite thin film system to induce the generation of periodic oxidation stripes. The transmission distance of the quasi-cylindrical waves is very short, only a few laser wavelengths, so the induced periodic structure formation is similar to an epitaxial growth process. Only one parallel oxide nanorod is formed at a time alongside the already formed periodic stripes. Through continuous epitaxial growth, the resulting periodic stripe structure is highly regular.

[0008] The metal-semiconductor composite thin film material described in this invention can achieve color tunability by varying the thickness of a semiconductor film with high absorption in the visible light band. Furthermore, single-beam femtosecond laser irradiation is used to induce the generation of a subwavelength periodic stripe structure, ultimately enabling the creation of rainbow-colored two-dimensional patterns on substrates of varying colors.

[0009] The present invention can prepare the metal-semiconductor composite thin film on-site, that is, by covering a semiconductor film on a metal film to form a metal-semiconductor composite thin film. The present invention can prepare the metal-semiconductor composite thin film material by magnetron sputtering or electron beam evaporation. Alternatively, an existing metal-semiconductor composite thin film product can be used.

[0010] The present invention needs to be carried out in the atmosphere or pure oxygen environment, and uses femtosecond laser to irradiate the composite film to induce an oxidation reaction in the semiconductor material, and forms periodically distributed one-dimensional oxide stripes in the laser irradiation area through the self-organization of the excited quasi-cylindrical waves.

[0011] When a femtosecond laser irradiates the surface of a metal-semiconductor composite film, a random oxide nanoparticle, on the order of hundreds of nanometers, forms at the center of the laser spot. Near-field enhancement occurs at both ends of the nanoparticle, perpendicular to the laser polarization. As a result, the nanorods gradually extend perpendicular to the polarization to form oxide nanorods. When the nanorods grow to a certain length, they scatter the incident laser light, stimulating quasi-cylindrical waves on the film surface. The wave vector of these quasi-cylindrical waves follows the polarization. Consequently, new oxide nanorods form on either side of the nanorod, within the region where the quasi-cylindrical waves and the laser light constructively interfere. Similarly, nanorods continue to extend outward from the center of the laser spot along the laser polarization. In the direction perpendicular to the laser polarization, the oxide nanorods also grow outward from the center of the laser spot due to near-field enhancement. Ultimately, a large-scale, regularly ordered, periodic stripe structure forms.

[0012] In the present invention, as a preference, the laser used is a femtosecond pulse laser, the spot energy distribution is Gaussian, the polarization state is linear polarization, the repetition frequency is unlimited, the wavelength is unlimited, the laser pulse width is less than 10 picoseconds, and the energy density at the focus is 0.01-0.05 J / cm 2 The laser is focused to the sample surface to induce oxidation reaction. The power density after focusing is much lower than the ablation threshold of the film. The multi-pulse ablation threshold of silicon film is 0.2J / cm 2 .

[0013] In the present invention, the lower layer of the composite film is a metal film, and the upper layer is a semiconductor film. The thickness of the metal film is not limited; preferably, the thickness of the semiconductor film is between 10-200 nanometers.

[0014] In the present invention, the metal material in the composite film is selected from a metal material with high loss in the optical band. The thickness of the metal layer is not strictly limited. Preferably, the metal film has a thickness of 50 to 200 nm. The material is selected from a metal with high absorption in the visible and near-infrared bands, such as one or more of titanium, tungsten, titanium nitride, nickel, molybdenum, chromium, gold, silver, copper, aluminum, platinum, etc.

[0015] In the present invention, the semiconductor material in the composite film is selected from materials that undergo oxidation reaction under femtosecond laser irradiation to form oxide nanoparticles. For example, the semiconductor material can be silicon, germanium, or a composite material containing either or both of silicon and germanium.

[0016] The nanoparticles formed by the present invention have diameters ranging from a few nanometers to hundreds of nanometers. Under continuous laser irradiation, the nanoparticles self-organize and accumulate through near-field interactions to form a one-dimensional nanograting structure. Preferably, the stripe width is 100-1000nm.

[0017] In the present invention, after laser irradiation, a raised periodic structure is formed on the smooth surface of the metal-semiconductor composite film. The period of the oxide stripes is related to the wavelength of the incident laser, but is always slightly smaller than the wavelength of the laser, close to 3 / 4 of the laser wavelength. The period of the oxide stripes is closely related to the incident angle and the thickness of the film, but has nothing to do with the optical properties of the metal material. Preferably, the period of the oxide stripes depends on the wavelength of the incident laser and / or the thickness of the structure on the surface; the periodic size of the periodically distributed oxide stripes can be controlled by adjusting one or more parameters among the laser incident wavelength, incident angle, and semiconductor film thickness. Preferably, the period of the periodically distributed oxide stripes is 500 to 1000 nm.

[0018] In the present invention, when the laser is incident normally, the orientation of the oxide stripes is perpendicular to the polarization direction of the incident femtosecond laser; therefore, when the laser is incident normally, the direction of the stripes can be controlled by adjusting the polarization direction; when the laser is incident obliquely, the stripe orientation is neither parallel nor perpendicular to the laser polarization, and the orientation of the oxide stripes depends on the laser incident direction.

[0019] In the present invention, the surface area of the oxide stripes is determined by the laser action area. The area and morphology of the fabricated grating can be controlled by controlling the incident light spot. The femtosecond laser action described above can be combined with a controllable two-dimensional translation stage to form the oxide stripes on the grating, enabling rapid, large-scale fabrication of grating structures.

[0020] Preferably, the present invention can use glass, sapphire, or silicon wafers with high flatness as the substrate, and deposit a silicon thin film (or other materials that meet the requirements) using vacuum magnetron sputtering (or other existing methods). The conditions of a highly flat film are more conducive to forming neat oxidation stripes.

[0021] For ease of processing, it is preferred that the femtosecond laser be focused on the sample in order to achieve a sufficiently high power density to oxidize the silicon surface.

[0022] Preferably, the polarization mode of the light spot needs to be determined before the femtosecond laser irradiates the film. The polarization state of the laser is determined by using a half-wave plate and a polarization beam splitter in combination with a light spot analyzer.

[0023] Preferably, during the process of femtosecond laser irradiation of the thin film, a spot analyzer is used to monitor the transmitted spot, so that the entire processing process can be monitored in real time.

[0024] As an optimal method, a large spot can be used to scan the film surface in two dimensions to achieve processing of any large area. Alternatively, when the laser energy is sufficient, the spot can be expanded to achieve single beam projection forming processing.

[0025] A thin film surface structure with periodic sub-wavelength nano-stripes is prepared by the method described in any of the above technical solutions.

[0026] The present invention provides a method for preparing a one-dimensional grating, which is prepared using any of the above-mentioned methods for manufacturing a periodic oxidation stripe structure. The method of the present invention can achieve rapid processing and preparation in a wider range.

[0027] The present invention provides a grating structure, which is prepared by the method described in any of the above technical solutions, and the grating structure is a grating structure with a sub-wavelength period.

[0028] The grating structure obtained by this invention, when illuminated by white light and observed from different angles, displays vibrant rainbow colors due to grating diffraction. The grating obtained by this invention can be applied to packaging applications such as digital products, cosmetics, food, alcohol and tobacco, publication covers, luxury goods, and infant products; labeling applications such as general labels, sealing tabs, and anti-counterfeiting labels; specialty materials such as general decorative shading, festive items, special laminations or transfers, and anti-counterfeiting pull lines; and personalized jewelry customization.

[0029] A mask structure is prepared by the method described in any of the above technical solutions.

[0030] In the present invention, the oxidized stripe grating structure formed by femtosecond laser induction can also be used as a mask in the future. Different thin films can be plated on its surface to form grating structures of different thin films.

[0031] A device for preparing a periodic oxidation stripe structure, comprising:

[0032] A femtosecond laser transmitter, used to provide the required femtosecond laser;

[0033] Light intensity adjustment element, which adjusts the energy of the input laser;

[0034] Polarization adjustment optical element, which adjusts the polarization of the incident laser to the desired polarization state;

[0035] Spot analyzer, used to observe the required laser spot pattern.

[0036] The lens element focuses the laser light after adjusting the light intensity and polarization direction and makes it incident on the silicon film.

[0037] When the polarization direction of the incident laser has been determined and meets the processing requirements, that is, when the polarization direction of the incident laser is consistent with the direction of the stripes to be processed, the above-mentioned polarization adjustment optical element can also be omitted.

[0038] When the processing conditions (laser irradiation time, etc.) are predetermined, the spot analyzer may be omitted.

[0039] The light intensity adjustment element generally includes an optical half-wave plate and an optical analyzer, and can also be an attenuation plate that can adjust the energy level, which is used to adjust the laser light intensity to obtain the laser with the energy we need.

[0040] Preferably, it also includes an image acquisition industrial camera for adjusting the spatial position of the laser spot acting on the surface of the semiconductor composite film and for observing the laser action process.

[0041] During the manufacturing process, laser light is emitted from a laser source, modulated by polarization-modulating optics, and then focused by a lens onto the sample. During laser irradiation, the intensity of the scattered light gradually changes as the oxidation stripes form and grow. Laser irradiation is terminated when the transmitted light intensity remains constant within a specified timeframe (determined by the laser repetition rate).

[0042] The present invention first uses a vacuum sputtering device to deposit a 50-200nm thick metal film (such as titanium nitride) on a sapphire substrate, followed by a 10-200nm thick semiconductor film (such as silicon). The sample is then irradiated with a femtosecond pulsed laser, causing oxidation and the gradual growth of periodic stripes. During the fabrication process, a spot analyzer monitors the transmitted laser spot, enabling real-time monitoring of the formation of the oxidized stripe grating structure.

[0043] In the present invention, the outstanding advantage of using femtosecond laser to prepare the grating structure is that an oxidized stripe structure with extremely high collimation can be obtained by irradiating only a single beam of femtosecond laser. No additional processing technology is required. It is direct, simple, clear and controllable. At the same time, the energy density required to produce the oxidized stripe grating structure is lower than that of long pulse and continuous lasers, and is far lower than the ablation threshold of the surface, which can avoid the formation of a large amount of ablation debris on the surface of the film. Therefore, the invention can realize the precision processing of large-area sub-wavelength nano-stripe structures. In addition, the period of the oxidized stripe grating structure is proportional to the incident wavelength. The grating period can be simply controlled by manipulating the incident laser wavelength, and the grating structure is in the sub-wavelength range, which will greatly simplify the process of high-precision micro-nano processing.

[0044] The present invention utilizes femtosecond lasers to directly illuminate micro- and nano-stripe structures with subwavelength periods. This represents a novel mechanism compared to laser direct writing, two-photon polymerization, interference lithography, or nanoimprinting. This method offers significant advantages in being more direct, simpler, and more efficient. The subwavelength period of the striations significantly reduces processing time, difficulty, and cost.

[0045] In addition, the presence of the metal layer in the present invention further reduces the threshold of the laser oxidation reaction; and also makes the oxidation stripes more regular. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 Schematic diagram of the composition of the equipment used in the embodiment for preparing a periodic oxidation stripe structure. Figure 2 The central wavelength is 1030 nm, the repetition rate is 5000 Hz, the pulse width is 130 fs, the spot size at the focus is 120 μm, and the energy density is 0.015 J / cm 2 A regular nanograting structure formed after 20 seconds of femtosecond laser irradiation on a composite thin film system consisting of 100nm titanium nitride and 50nm amorphous silicon thin films. The arrow indicates the direction of laser polarization.

[0047] Figure 3 This is an energy-dispersive X-ray spectrum of the two-dimensional distribution of oxygen in a nanograting induced by a femtosecond laser. Bright areas indicate high oxygen content.

[0048] Figure 4 To observe the oxidation in composite films by cutting nanogratings using a focused ion beam.

[0049] Figure 5 In-situ observation of the epitaxial growth process of periodic nanostripes under an optical microscope. N represents the number of irradiation pulses.

[0050] Figure 6 This is the stripe structure induced by femtosecond laser on titanium nitride plus 50 nanometer amorphous silicon film at an incident angle of 60°. DETAILED DESCRIPTION

[0051] The present invention will be further described below in conjunction with the accompanying drawings:

[0052] like Figure 1As shown, a device for inducing the formation of a self-organized periodic oxidation stripe structure on the surface of a metal-semiconductor composite film using a femtosecond laser. It includes a half-wave plate 5, an analyzer 6, a half-wave plate 7, a focusing lens 8 and an industrial camera 9. The optical elements half-wave plate 5 and analyzer 6 are used to continuously change the energy of the laser. The repetition frequency of the femtosecond laser is not limited, and the repetition frequency in this example is 5000 Hz. The half-wave plate 7 is used to adjust the polarization direction of the laser incident on the sample surface. The laser light 4 emitted by the femtosecond laser is focused by a lens 8 onto a composite film adhered to a substrate 1, wherein the film 2 is a metal material and the film 3 is a semiconductor material. In this example, the metal is titanium nitride and the semiconductor is silicon.

[0053] In this example, the focal length of lens 8 is 20 cm. The femtosecond laser has a Gaussian spot with a central wavelength of 1030 nm, a pulse width of 130 fs, and a spot diameter of 120 μm at the focal point. In this example, a 100-nm thick titanium nitride film and a 50-nm thick amorphous silicon film were deposited on a 500-μm thick sapphire substrate using a magnetron sputtering device.

[0054] During laser-induced self-organized nanostripe processing, when a low-energy laser is focused onto the flat surface of a silicon membrane, it is primarily reflected by the membrane, resulting in virtually no scattered light being detected by the lateral industrial camera 9. By rotating the half-wave plate 5 to gradually increase the incident laser energy until the silicon's oxidation threshold is reached, a small amount of oxide particles appear on the surface of the silicon membrane. At this point, the industrial camera 9 can observe the increase in scattered light in real time. The incident laser energy is maintained constant, and changes in the scattered light spot captured by the industrial camera 9 are monitored. As the number of irradiation pulses gradually increases, the scattered light spot essentially ceases to change, and laser irradiation is stopped. A scanning electron microscope can then be used to observe the striped structure, which exhibits periodic changes in scale.

[0055] like Figure 2 As shown, in this example, the energy density is 0.015J / cm 2 After irradiating the composite film with a femtosecond laser for 20 seconds, a high-resolution scanning electron microscope was used to observe the formation of a neat and orderly grating structure with a period of 730 nanometers on the silicon film. Its orientation was perpendicular to the polarization direction of the laser. There was no significant difference in the period formed in the center and edge areas of the spot, which means that the change in refractive index caused by the femtosecond laser is not important for the stripe formation process. Figure 3 As shown, the element analysis of the laser irradiation area was performed using an energy dispersive X-ray spectrometer, and it was found that the periodic stripes in the formed grating structure (i.e. Figure 3 The light-colored area in the middle contains a large amount of oxygen, which proves that the grating structure is produced by laser-induced oxidation reaction rather than ablation effect. Figure 4As shown, further cross-sectional observation of the grating structure using a focused ion beam revealed that the oxidation process is confined to the silicon film, while the underlying titanium nitride metal layer remains unaffected. This is because silicon has a lower oxidation threshold than titanium nitride. The cross-sectional image further reveals that the periodic stripes formed are the result of a large accumulation of nanoparticles.

[0056] Figure 5 The growth process of periodic nanostripes was observed in situ using an optical microscope. At the focal point, a nanoparticle is first formed, which gradually extends to form a nanorod due to near-field enhancement. At the same time, the scattering of the nanorod interferes with the incident laser, forming a periodic interference pattern, which leads to the epitaxial growth of the nanorod. Finally, a large area of periodic, neat and orderly stripes is gradually formed. Figure 6 As shown in FIG, when the laser is incident at an angle, the generated stripes are neither parallel nor perpendicular to the polarization direction, and their period is 660 nm, which is significantly smaller than the period at normal incidence.

Claims

1. A method for producing ordered sub-wavelength nano-stripes using femtosecond laser, characterized in that: include: The semiconductor film side of the metal-semiconductor composite film is irradiated with a femtosecond laser, inducing an oxidation reaction in the semiconductor material to form oxide particles, which are further formed under the action of the near-field enhancement effect. Finally, under the induction of the quasi-cylindrical wave excited by the laser, periodically distributed oxide stripes are self-organized. The lower layer of the composite film is a metal film, and the upper layer is a semiconductor film, and the thickness of the semiconductor film is between 10-200 nanometers; In the composite film, the metal material is selected from titanium nitride; The semiconductor material is selected from silicon; The thickness of the metal film is 50-200 nm.

2. The method for producing ordered sub-wavelength nano-stripes using femtosecond laser according to claim 1, characterized in that: The spot energy distribution is Gaussian, the polarization state is linear polarization, and the energy density at the focus is 0.01~0.05 J / cm 2 between.

3. The method for producing ordered sub-wavelength nano-stripes using femtosecond laser according to claim 1, characterized in that: When the laser is incident at right angles, the orientation of the oxide stripes is perpendicular to the polarization direction of the incident femtosecond laser; when the laser is incident at oblique angles, the orientation of the oxide stripes is related to the incident direction of the laser.

4. The method for producing ordered sub-wavelength nano-stripes using femtosecond laser according to claim 1, characterized in that: The periodic size of the periodically distributed oxide stripes can be controlled by adjusting one or more parameters among the laser incident wavelength, incident angle, and semiconductor film thickness.

5. A nanograting, characterized in that: The method according to any one of claims 1 to 4 is used for preparation.

6. A thin film surface structure with periodic sub-wavelength nano-stripes, characterized in that: The method according to any one of claims 1 to 4 is used for preparation.

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